Reactivity of [(C5Me5)Fe(C5H4BBr2)] and [(OC ... - ACS Publications

Abbreviations: s = singlet, d = doublet, tr = triplet, q = quartet, vtr = virtual triplet, br = broad, n.r. = multiplet expected in the 1H NMR spectru...
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Organometallics 2010, 29, 5301–5309 DOI: 10.1021/om1004513

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Reactivity of [(C5Me5)Fe(C5H4BBr2)] and [(OC)3Mn(C5H4BBr2)] toward Et3SiH: Facile Access to [(OC)3Mn(C5H4BH2)]2 and to Boron-Bridged Dinuclear Organometallics† U. David Eckensberger, Mitra Weber, Julia Wildt, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner* Institut f€ ur Anorganische und Analytische Chemie, J. W. Goethe-Universit€ at Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt (Main), Germany Received May 10, 2010

The reaction of Fc#BBr2 with 2 equiv of Et3SiH gives the boron-bridged dinuclear species Fc#2BBr with concomitant liberation of B2H6 (Fc# = (C5Me5)Fe(C5H4)). The transformation is slow (48 h) but nevertheless provides yields of >85%. Fc#2BBr can conveniently be converted into the aminoborane Fc#2BNMe2 by treatment with Me3SiNMe2. For an X-ray crystal structure analysis, Fc#2BNMe2 was hydrolyzed to the borinic acid Fc#2BOH. A cross-coupling experiment using Et3SiH and an equimolar mixture of FcBBr2 and Fc#BBr2 led to the formation of the mixed product Fc(Fc#)BBr with high selectivity (Fc = (C5H5)Fe(C5H4)). Similar to the case for Fc#2BBr, Cym2BBr can be prepared from CymBBr2 and Et3SiH (Cym = (OC)3Mn(C5H4)). In contrast to FcBH2, which exists only as transient intermediate on the way to Fc2BH, CymBH2 is an isolable species. The compound was synthesized from a concentrated toluene solution of CymBBr2 and excess neat Et3SiH at -78 °C and forms B-H-B-bridged dimers (CymBH2)2 in the solid state (X-ray crystallography). Our reactivity studies indicate that the Et3SiH-induced coupling reaction of derivatives LnM(C5H4BBr2) is fastest for electron-rich cyclopentadienyl complexes. Addition of NMe2Et to (CymBH2)2 affords the monomeric amine adduct CymBH2(NMe2Et). The reaction of (CymBH2)2 with HCCtBu in a stoichiometric ratio of 1:4 (or slightly higher) results in a mixture of products from which the dinuclear hydroboration product (CymB(C(H)dC(H)tBu))2C(H)-C(H)2tBu could be isolated and structurally characterized. To obtain the divinylborane CymB(C(H)dC(H)tBu)2 in high yield and pure form, (CymBH2)2 has to be dissolved in excess neat HCCtBu. Introduction Organoboranes have various applications as catalysts, sensor systems, or luminescent materials,1-5 and their useful properties can be expanded further by the incorporation of transition-metal ions into the molecular frameworks.6-26

In this context, our group has recently discovered a convenient condensation reaction leading from mono- to diferrocenylboranes and from 1,10 -diborylferrocenes to boranediyl-bridged poly(ferrocenylene)s (Scheme 1).9,27-31 Two ways of conducting this (poly)condensation do currently exist: starting from the ferrocenylborohydride A or B, treatment with Me3SiCl leads to the abstraction of one

† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar Seyferth with gratitude and respect. *To whom correspondence should be addressed. Fax: þ49 69 798 29260. E-mail: [email protected]. (1) Entwistle, C. D.; Marder, T. B. Angew. Chem., Int. Ed. 2002, 41, 2927–2931. (2) Hoefelmeyer, J. D.; Schulte, M.; Tschinkl, M.; Gabbaı¨ , F. P. Coord. Chem. Rev. 2002, 235, 93–103. (3) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004, 16, 4574–4585. (4) J€ akle, F. Coord. Chem. Rev. 2006, 250, 1107–1121. (5) J€ akle, F. Chem. Rev. 2010, DOI: 10.1021/cr100026f. (6) Ma, K.; Scheibitz, M.; Scholz, S.; Wagner, M. J. Organomet. Chem. 2002, 652, 11–19. (7) Gabbaı¨ , F. P. Angew. Chem., Int. Ed. 2003, 42, 2218–2221. (8) Aldridge, S.; Bresner, C. Coord. Chem. Rev. 2003, 244, 71–92. (9) Wagner, M. Angew. Chem., Int. Ed. 2006, 45, 5916–5918. (10) Hudson, Z. M.; Wang, S. Acc. Chem. Res. 2009, 42, 1584–1596. (11) Kuhlmann, T.; Roth, S.; Roziere, J.; Siebert, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 105–107. (12) Lavrentiev, M. Y.; K€ oppel, H.; B€ ohm, M. C. Chem. Phys. 1993, 169, 85–102. (13) M€ uller, P.; Pritzkow, H.; Siebert, W. J. Organomet. Chem. 1996, 524, 41–47.

(14) Dusemund, C.; Sandanayake, K. R. A. S.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1995, 333–334. (15) Yamamoto, H.; Ori, A.; Ueda, K.; Dusemund, C.; Shinkai, S. Chem. Commun. 1996, 407–408. (16) Venkatasubbaiah, K.; Zakharov, L. N.; Kassel, W. S.; Rheingold, A. L.; J€akle, F. Angew. Chem., Int. Ed. 2005, 44, 5428–5433. (17) Venkatasubbaiah, K.; Nowik, I.; Herber, R. H.; J€akle, F. Chem. Commun. 2007, 2154–2156. (18) Venkatasubbaiah, K.; Doshi, A.; Nowik, I.; Herber, R. H.; Rheingold, A. L.; J€akle, F. Chem. Eur. J. 2008, 14, 444–458. (19) Pakkirisamy, T.; Venkatasubbaiah, K.; Kassel, W. S.; Rheingold, A. L.; J€akle, F. Organometallics 2008, 27, 3056–3064. (20) Melaimi, M.; Gabbaı¨ , F. P. J. Am. Chem. Soc. 2005, 127, 9680–9681. (21) J€akle, F.; Lough, A. J.; Manners, I. Chem. Commun. 1999, 453–454. (22) Gamboa, J. A.; Sundararaman, A.; Kakalis, L.; Lough, A. J.; J€akle, F. Organometallics 2002, 21, 4169–4181. (23) Boshra, R.; Sundararaman, A.; Zakharov, L. N.; Incarvito, C. D.; Rheingold, A. L.; J€akle, F. Chem. Eur. J. 2005, 11, 2810–2824. (24) Boshra, R.; Venkatasubbaiah, K.; Doshi, A.; Lalancette, R. A.; Kakalis, L.; J€akle, F. Inorg. Chem. 2007, 46, 10174–10186.

r 2010 American Chemical Society

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Scheme 1. Reactions of Ferrocenylhydridoboranes and -boratesa

Figure 1. Computed structure of a key intermediate of the condensation reaction 2FcBBr2 þ 3Et3SiH f Fc2BBr þ 0.5B2H6 þ 3Et3SiBr. Scheme 2. Synthesis of 2-5a

a Legend: (i) n = 1, A þ excess Me3SiCl/Do in Et2O (Do = NMe2Et) or SMe2 (Do = SMe2); (ii) n = 2, A þ excess Me3SiCl in Et2O; (iii) B þ excess Me3SiCl in Me2S; (iv) n = 2, F þ 3Et3SiH in toluene; (v) G þ 3nEt3SiH in toluene.

hydride ion per borohydride unit with the intermediate formation of FcBH2 or H2B-fc-BH2, respectively (Fc = (C5H5)Fe(C5H4); fc = Fe(C5H4)2). In the presence of suitable Lewis bases (Do = NMe2Et, SMe2), the generated boranes can be trapped as their Lewis acid-base adducts C and (Do)H2B-fcBH2(Do).30 In the absence of donor molecules, neither FcBH2 nor H2B-fc-BH2 is stable; they undergo dimerization (D) or polymerization (E) with liberation of B2H6.27,30 Alternatively, one can start from dibromoborylferrocenes F and G, which, upon reaction with Et3SiH, give dinuclear (25) (a) Broomsgrove, A. E. J.; Addy, D. A.; Bresner, C.; Fallis, I. A.; Thompson, A. L.; Aldridge, S. Chem. Eur. J. 2008, 14, 7525–7529. (b) Day, J. K.; Bresner, C.; Coombs, N. D.; Fallis, I. A.; Ooi, L.-L.; Aldridge, S. Inorg. Chem. 2008, 47, 793–804. (26) (a) Braunschweig, H.; Radacki, K.; Rais, D.; Seeler, F. Organometallics 2004, 23, 5545–5549. (b) Braunschweig, H; Radacki, K.; Rais, D.; Scheschkewitz, D. Angew. Chem., Int. Ed. 2005, 44, 5651–5654. (c) Braunschweig, H.; Kraft, M.; Schwarz, S.; Seeler, F; Stellwag, S. Inorg. Chem. 2006, 45, 5275–5277. (d) Braunschweig, H.; Leech, R.; Rais, D.; Radacki, K.; Uttinger, K. Organometallics 2008, 27, 418–422. (27) Scheibitz, M.; Bats, J. W.; Bolte, M.; Lerner, H.-W.; Wagner, M. Organometallics 2004, 23, 940–942. (28) Heilmann, J. B.; Scheibitz, M.; Qin, Y.; Sundararaman, A.; J€ akle, F.; Kretz, T.; Bolte, M.; Lerner, H.-W.; Holthausen, M. C.; Wagner, M. Angew. Chem., Int. Ed. 2006, 45, 920–925. (29) Heilmann, J. B.; Qin, Y.; J€akle, F.; Lerner, H.-W.; Wagner, M. Inorg. Chim. Acta 2006, 359, 4802–4806.

a Legend: (i) þ4.4Et3SiH in toluene, room temperature, 48 h; (ii) þ3Me3SiNMe2 in toluene, room temperature, 1 h; (iii) þ excess H2O in toluene, room temperature, 16 h; (iv) þ4.4Et3SiH in toluene, room temperature, 30 h.

(H) and polynuclear (I) compounds, respectively.28 Again, B2H6 is ultimately formed as the byproduct. Polymers E and I can easily be transformed into various neutral or polycationic derivatives via hydroboration, ether cleavage, or nucleophilic substitution protocols.28-31 In the case of the reaction F f H, we have thoroughly investigated the potential energy surface using DFT calculations.28 A dimer with one bridging hydride ion and one bridging ferrocenyl moiety was identified as a key intermediate structure (Figure 1). This finding raised questions concerning the steric and electronic requirements that have to be met to make the formation of such a species favorable. With the aim of exploring the limitations of the condensation reaction and to extend its scope to organometallic compounds other than parent ferrocene, we have now investigated the reaction of Et3SiH with Fc#BBr2 (1; Scheme 2) (30) Scheibitz, M.; Li, H.; Schnorr, J.; Sanchez Perucha, A.; Bolte, M.; Lerner, H.-W.; J€akle, F.; Wagner, M. J. Am. Chem. Soc. 2009, 131, 16319–16329. (31) Cui, C.; Heilmann-Brohl, J.; Sanchez Perucha, A.; Thomson, M. D.; Roskos, H. G.; Wagner, M.; J€akle, F. Macromolecules 2010, DOI: 10.1021/ma100440g.

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Table 1. Selected Crystallographic Data for 1, 4 3 H2O, and (7)2 formula fw color, shape temp (K) radiation, λ (A˚) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd (g cm-3) F(000) μ (mm-1) cryst size (mm3) no. of rflns collected no. of indep rflns (Rint) no. of data/restraints/params GOF on F2 R1, wR2 (I > 2σ(I)) R1, wR2 (all data) largest diff peak and hole (e A˚-3)

1

4 3 H2O

(7)2

C15H19BBr2Fe 425.78 red, plate 173(2) Mo KR, 0.710 73 triclinic P1 7.2326(9) 8.6620(11) 13.1522(17) 92.874(11) 91.017(11) 106.655(10) 787.96(17) 2 1.795 420 6.011 0.42  0.19  0.12 12 147 2782 (0.0714) 2782/0/178 0.928 0.0479, 0.1114 0.0717, 0.1206 0.632, -1.258

C30H39BFe2O 3 H2O 556.14 orange, plate 173(2) Mo KR, 0.710 73 triclinic P1 8.2304(6) 12.4285(9) 13.8890(10) 98.643(6) 94.324(6) 90.250(6) 1400.42(18) 2 1.319 588 1.060 0.37  0.34  0.13 23 519 5216 (0.0415) 5216/3/335 1.037 0.0340, 0.0896 0.0408, 0.0928 0.514, -0.414

C16H12B2Mn2O6 431.76 colorless, needle 173(2) Mo KR, 0.710 73 monoclinic P21/c 6.6618(9) 11.1029(11) 12.2334(18) 90 102.100(11) 90 884.7(2) 2 1.621 432 1.456 0.19  0.09  0.08 7395 1651 (0.0925) 1651/0/126 0.949 0.0402, 0.0656 0.0727, 0.0721 0.268, -0.352

Scheme 3. Synthesis of (7)2-9a

a Legend: (i) þ excess Et3SiH, -78 °C, 5 min; (ii) þ3Et3SiH in toluene, room temperature, 20 h; (iii) þ3Me3SiNMe2 in toluene, room temperature, 2 h.

on the one hand and CymBBr2 (6; Scheme 3) on the other (Fc# = (C5Me5)Fe(C5H4); Cym = (OC)3Mn(C5H4)). The reason for this selection is that Fc#BBr2 possesses a significantly higher steric demand than FcBBr2, whereas CymBBr2 bears a less electron-rich organometallic substituent.

Results and Discussion Synthesis and Characterization of Bis(pentamethylferrocenyl)boranes. The starting material, Fc#BBr2 (1), has already been mentioned in the literature;25 however, no detailed synthesis protocol and no NMR spectroscopic characterization was provided. This information, together with a description of the dimethylamino derivative Fc#B(NMe2)2, is available in the Supporting Information. (32) Appel, A.; J€ akle, F.; Priermeier, T.; Schmid, R.; Wagner, M. Organometallics 1996, 15, 1188–1194.

Figure 2. Molecular structure and numbering scheme of 1. Displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths (A˚), bond angles (deg), torsion angles (deg), and dihedral angles (deg): B(1)-Br(1) = 1.933(7), B(1)Br(2) = 1.931(7), B(1)-C(1) = 1.507(9); Br(1)-B(1)-Br(2) = 116.2(4), Br(1)-B(1)-C(1) = 121.2(5), Br(2)-B(1)-C(1) = 122.6(5), COG-Fe(1)-COG0 = 173.4; C(1)-COG-COG0 C(12) = 11.9; B(1)Br(1)Br(2)//Cp = 10.9; R* = 12.3. COG and COG0 denote centroids of the cyclopentadienyl rings.

In order to get an estimate of the degree of steric crowding in 1, its solid-state structure was determined by X-ray crystallography (Table 1 and Figure 2). In comparison to the parent compound FcBBr2,32 which crystallizes with two crystallographically independent molecules in the asymmetric unit, the following differences between the molecular structures are evident. (i) The B-Cp bond is longer by about 0.03 A˚ in 1 than in FcBBr2. (ii) The dip angle R* equals 17.7°/18.9° in FcBBr2 but only 12.3° in 1 (R* = 180° - R, R = B-CipsoCOG; COG = centroid of the cyclopentadienyl ring). It has

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been shown that R* can be taken as a measure of Fe 3 3 3 B through-space binding in borylated ferrocenes.32-34 Consequently, large values of R* are usually observed for strongly Lewis acidic boryl substituents in combination with electronrich ferrocenyl moieties. While the Lewis acidities of the BBr2 fragments are the same in 1 and FcBBr2, pentamethylferrocene is more electron-rich than ferrocene. Consequently, the smaller R* value of 1 cannot be explained by electronic arguments but has to be attributed to a higher degree of steric crowding in this molecule. Consistent with this interpretation, we also observe a slightly more pronounced bending of the ferrocene backbone in 1 (COG-Fe(1)COG0 = 173.4°) as compared to FcBBr2 (175.3°/175.9°). According to 11B{1H} NMR spectroscopy, the formation of Fc#2BBr (2) from 1 and Et3SiH (Scheme 2) is at least 10 times slower than in the case of Fc2BBr (H). 2 gives rise to a signal at δ(11B) 58.1 (h1/2 = 550 Hz), which is a downfield shift of 8.2 ppm compared to the 11B resonance of the starting material 1 (δ(11B) 49.9; h1/2 = 200 Hz). Similar shift differences have already been observed between the 11B signals of Fc2BBr (H; 55.3 ppm)33 and FcBBr2 (F; 46.7 ppm).35 The 1H NMR spectrum of 2 shows one signal for the methyl groups (1.73 ppm) and two broad signals for the C5H4 rings (4.08, 4.43 ppm). For further characterization, 2 was transformed into the less air- and moisture-sensitive dimethylamino derivative 3 by treatment with Me3SiNMe2 in toluene (Scheme 2). The 11 B resonance of 3 appears at 41.6 ppm, which is within the typical shift range36 of mono(amino)diorganylboranes (cf. Fc2B(NMe2): δ(11B) 41.037). The proton integral ratios in the 1H NMR spectrum of 3 provide further evidence for the presence of two Fc# moieties in the molecule (NMe2: C5Me5:C5H4 = 6H:30H:8H). Moreover, a MALDI mass spectrum of 3 showed one peak at m/z 565, which is the required value for the ion [Fc#2BNMe2]þ. Neither 2 nor 3 gave single crystals suitable for X-ray crystallography. Thus, 3 was deliberately hydrolyzed to the borinic acid 4 (Scheme 2), which finally crystallized from benzene together with 1 equiv of H2O (4 3 H2O; Table 1 and Figure 3). In the solid state, 4 3 H2O forms centrosymmetric dimers which are held together by hydrogen bonds. Each water molecule acts as a 2-fold hydrogen bond donor: toward the BOH group of one Fc#2BOH molecule and toward the cyclopentadienyl π-electron system of the other Fc#2BOH molecule (a plot of the supramolecular structure is shown in the Supporting Information). The boron atom of 4 3 H2O possesses a trigonal-planar configuration and is well shielded from the bottom and the top by two methyl groups (B(1) 3 3 3 C(26) = 3.394(4) A˚, B(1) 3 3 3 C(36) = 3.381(5) A˚). We observe significant differences between the individual bond angles about B(1), which are indicative of substantial intra(33) Scheibitz, M.; Bolte, M.; Bats, J. W.; Lerner, H.-W.; Nowik, I.; Herber, R. H.; Krapp, A.; Lein, M.; Holthausen, M. C.; Wagner, M. Chem. Eur. J. 2005, 11, 584–603. (34) Kaufmann, L.; Vitze, H.; Bolte, M.; Lerner, H.-W.; Wagner, M. Organometallics 2008, 27, 6215–6221. (35) Renk, T.; Ruf, W.; Siebert, W. J. Organomet. Chem. 1976, 120, 1–25. (36) N€ oth, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectroscopy of Boron Compounds. In NMR Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer: Berlin, Heidelberg, New York, 1978. (37) Haghiri Ilkhechi, A.; Bolte, M.; Lerner, H.-W.; Wagner, M. J. Organomet. Chem. 2005, 690, 1971–1977.

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Figure 3. Molecular structure and numbering scheme of 4 3 H2O. Displacement ellipsoids are drawn at the 50% probability level; H atoms (except the OH group) and the H2O molecule are omitted for clarity. Selected bond lengths (A˚), bond angles (deg), and dihedral angles (deg): B(1)-O(1) = 1.380(3), B(1)-C(1) = 1.554(3), B(1)-C(11) = 1.560(3); O(1)-B(1)-C(1) = 118.1(2), O(1)-B(1)-C(11) = 116.6(2), C(1)-B(1)-C(11) = 125.4(2); Cp(C(1))//Cp(C(11)) = 0.5, Cp(C(1))//O(1)B(1)C(1)C(11) = 6.2, Cp(C(11))//O(1)B(1)C(1)C(11) = 6.5, Cp(C(1))//Cp*(C(21)) = 2.6, Cp(C(11))//Cp*(C(31))=3.0; R*(C(1))=6.1, R*(C(11))=6.0.

molecular steric strain (cf. O(1)-B(1)-C(1)/C(11) = 118.1(2)°/ 116.6(2)° vs C(1)-B(1)-C(11) = 125.4(2)°). The B(1)O(1) = 1.380(3) A˚ bond length is almost identical with the BO distances expected for three-coordinate boron38 and lies in the same range as those of Mes2BOH (two crystallographically independent molecules in the asymmetric unit, B-O = 1.367(6)/1.369(4) A˚; Mes = 2,4,6-Me3C6H2).39 Looking at the calculated reaction mechanism,28 it is evident that the relative energy of the productive intermediate structure (Figure 1) with respect to the nonproductive dimer Fc(Br)B-(μ-H)2-B(Br)Fc is an important factor governing the efficiency of the condensation reaction. The bulkiness of a pentamethylcyclopentadienyl ring, especially when it resides on the bridging ferrocene fragment, should disfavor the productive intermediate. The positive inductive effect of the five methyl groups, however, is likely to stabilize it. These inverse factors raise the question whether the reaction of Et3SiH with a 1:1 mixture of FcBBr2 (F) and Fc#BBr2 (1) will lead to the preferred formation of the unsymmetric condensation product Fc(Fc#)BBr. Apart from a shorter reaction time, the experiment was carried out under the same conditions as the homocoupling reaction 1 f 2 and indeed gave >80% of Fc(Fc#)BBr (5; Scheme 2) and only trace quantities of Fc2BBr (H) and Fc#2BBr (2). Since it was not possible to separate H, 2, and 5 from one another, the relative amount of 5 had to be determined by 1H NMR spectroscopy (all resonances assigned to 5 were identical with those of an authentic sample prepared from FcSnBu340 and Fc#BBr2 (1)). Synthesis and Characterization of Mono- and Dicymantrenylboranes. The remarkable selectivity with which the mixed (38) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements. Pergamon: Oxford, England, 1984; p 233. (39) Weese, K. J.; Bartlett, R. A.; Murray, B. D.; Olmstead, M. M.; Power, P. P. Inorg. Chem. 1987, 26, 2409–2413. (40) Guillaneux, D.; Kagan, H. B. J. Org. Chem. 1995, 60, 2502–2505.

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Figure 4. Molecular structure and numbering scheme of (7)2. Displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths (A˚), atom 3 3 3 atom distances (A˚), bond angles (deg), and dihedral angles (deg): B(1)-C(11) = 1.563(6), Mn(1)-COav = 1.799(4), C-Oav = 1.156(5), B(1)-H(1A) = 1.09(4), B(1)-H(1B) = 1.30(3), B(1)-H(1B$) = 1.27(4), B(1) 3 3 3 B(1$) = 1.783(8), Mn(1) 3 3 3 COG = 1.780; B(1)-C(11)-C(12) = 129.8(3), B(1)-C(11)-C(15) = 124.7(3), B(1)-H(1B)-B(1$) = 88(2), H(1B)-B(1)-H(1B$) = 92(2); B(1$) 3 3 3 B(1)-C(11) = 122.3(4); Cp(C(11))//Cp(C(11$)) = 0; R* = 3.4. COG = centroid of the cyclopentadienyl ring. Symmetry transformation used to generate equivalent atoms: ($) -x þ 2, -y þ 2, -z þ 1.

condensation product Fc(Fc#)BBr (5) is formed indicates that the intermediate structure (Figure 1) gains more from the electron-releasing capacity of five methyl groups than it loses due to their steric demand. In turn, this supports our previous assumption that the substituent transfer is best achieved with electron-rich metallocenes. To further test this working hypothesis, we also investigated the reactivity of the electron-poor CymBBr2 (6; Cym = (OC)3Mn(C5H4))35 toward Et3SiH (Scheme 3). Addition of 6 in toluene to an excess of neat Et3SiH at -78 °C led to the immediate precipitation of a pale yellow solid, which was isolated by filtration while the reaction mixture was still cold. When the reaction is performed without stirring, the product can be obtained in single-crystal form. An X-ray crystal structure analysis revealed the centrosymmetric dimers (7)2 (Table 1, Figure 4) of cymantrenylborane, CymBH2 (7). These dimers are connected by two B-H-B two-electronthree-center bonds leading to a B 3 3 3 B distance of 1.783(8) A˚. Only two other examples of crystallographically characterized monoorganylboranes are known in the literature, and (41) Wehmschulte, R. J.; Diaz, A. A.; Khan, M. A. Organometallics 2003, 22, 83–92. (42) The crystal structure of [(Me3Si)3CBH(μ-H)]2 has been determined twice, and significantly different B 3 3 3 B distances have been reported: (a) Paetzold, P; Geret, L.; Boese, R. J. Organomet. Chem. 1990, 385, 1–11. (b) Al-Juaid, S. S.; Eaborn, C.; Hitchcock, P. B.; Kundu, K. K.; Molla, M. E.; Smith, J. D. J. Organomet. Chem. 1990, 385, 13–21.

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these have B 3 3 3 B distances of 1.787(5) A˚ ([2,6-(2-MeC6H4)2C6H3BH(μ-H)]2)41 and 1.747(14) A˚/1.839(11) A˚ ([(Me3Si)3CBH(μ-H)]2).42 The mutually trans-positioned cyclopentadienyl rings adopt a crystallographically imposed coplanar conformation; the two boron atoms are only slightly dislocated from this plane (R* = 3.4). The central B2H2 fourmembered ring possesses essentially a square geometry with B-H bond lengths of 1.27(4) and 1.30(3) A˚ and bond angles of B-H-B = 88(2)° and H-B-H = 92(2)°. As is to be expected, the B-H bond to the terminal hydrogen atom (B(1)-H(1A) = 1.09(4) A˚) is shorter than those to the bridging hydrogen atoms. An IR spectrum of (7)2 in KBr shows carbonyl absorptions at ν~ 2013 cm-1/1935 cm-1 as well as borane absorptions at ν~ 2552 cm-1 (B-H)/1547 cm-1 (B-H-B). NMR spectra of the compound were measured in THF-d8, because the use of nondonor solvents led to decomposition with gas evolution. In the 11B{1H} NMR spectrum, one singlet appears at 4.5 ppm, thereby testifying to the presence of four-coordinate boron nuclei.36 In the corresponding proton-coupled spectrum, the resonance splits into a triplet with a coupling constant of 1JBH = 91 Hz. Signals at δ(1H) 4.66, 4.74 (2  vtr) and δ(13C) 84.5, 91.0 are assignable to the cymantrenyl group. These findings point toward a dissociation of the dimer in solution with formation of the THF adduct CymBH2(THF). So far, the major message of our CymBBr2/Et3SiH reactivity studies is that CymBH2 (7) can be synthesized and isolated as the dimeric entity (7)2. In contrast to that, the corresponding ferrocenylborane FcBH2 is only a transient intermediate on the way to Fc2BH (D; Scheme 1). This result fits nicely to our a priori expectations regarding the behavior of dibromoboranes with electron-poor and electron-rich organometallic substituents in the condensation reaction. The next question to be asked was whether it is at all possible to perform the condensation reaction with CymBBr2 (6) or whether the activation energy is prohibitively high, because the cymantrenyl ring refuses to adopt a bridging position between two electrophilic boron atoms. To answer this question, we treated a toluene solution of 6 with only 1.5 equiv of Et3SiH; the mixture was stirred for 20 h at room temperature and monitored by 11B{1H} NMR spectroscopy (Scheme 3). As the time progressed, the signal of the starting material (δ(11B) 49.2)35 gradually vanished and a new resonance appeared instead (δ(11B) 53.2). The direction and the magnitude of this low-field shift (Δδ = 4.0) corresponds reasonably well to the changes in the 11B{1H} NMR spectra during the transitions FcBBr2 f Fc2BBr (Δδ = 8.6)33,35 and Fc#BBr2 f Fc#2BBr (Δδ = 8.2). After workup, resonances assignable to cyclopentadienyl ligands were found at δ(1H) 4.09, 4.80 and δ(13C) 87.0, 94.9. However, as in the case of Fc#2BBr, these data do not provide conclusive evidence for the presence of precisely two organometallic substituents in the product molecule. The bromo substituent of Cym2BBr (8) was therefore exchanged for a Me2N group with the help of Me3SiNMe2 (Scheme 3). Both the resulting 11B{1H} NMR spectrum (δ(11B) 36.7) and the proton integral ratios between the Me2N signal (δ(1H) 2.53; 6H) and the cyclopentadienyl resonances (δ(1H) 4.15, 4.35; 2  4H) confirm the identity of the product obtained with the mono(amino)dicymantrenylborane Cym2BNMe2 (9). Cym2BBr (8) and Cym2BNMe2 (9) have also been characterized by X-ray crystallography (cf. the Supporting Information for the crystal structure analysis of 9). The crystal

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Table 2. Selected Crystallographic Data for 8, 10, and 11

formula fw color, shape temp (K) radiation, λ (A˚) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd (g cm-3) F(000) μ (mm-1) cryst size (mm3) no. of rflns collected no. of indep rflns (Rint) no. of data/restraints/params GOF on F2 R1, wR2 (I > 2σ(I)) R1, wR2 (all data) largest diff peak and hole (e A˚-3)

8

10

11

C16H8BBrMn2O6 496.82 light yellow, plate 173(2) Mo KR, 0.710 73 monoclinic P21/c 12.3192(10) 12.6324(7) 22.7600(18) 90 96.661(6) 90 3518.0(4) 8 1.876 1936 3.736 0.13  0.09  0.02 19 331 6189 (0.1091) 6189/0/469 0.853 0.0543, 0.1016 0.1218, 0.1210 0.815, -0.679

C12H17BMnNO3 289.02 colorless, block 173(2) Mo KR, 0.710 73 monoclinic P21/n 10.9481(5) 11.1176(6) 11.3690(5) 90 95.951(3) 90 1376.34(11) 4 1.395 600 0.957 0.30  0.28  0.22 24 429 2680 (0.0850) 2680/0/163 0.916 0.0324, 0.0689 0.0505, 0.0730 0.337, -0.450

C34H42B2Mn2O6 678.18 yellow, block 173(2) Mo KR, 0.710 73 tetragonal I41/a 19.7936(14) 19.7936(14) 35.745(4) 90 90 90 14004(2) 16 1.287 5664 0.762 0.26  0.25  0.22 17 661 6561 (0.1146) 6561/18/395 0.943 0.0664, 0.1519 0.1072, 0.1689 0.834, -0.813

Figure 5. Molecular structure and numbering scheme of 8A. Displacement ellipsoids are drawn at the 30% probability level. Selected bond lengths (A˚), bond angles (deg), and dihedral angles (deg): B(1)-Br(1) = 1.951(10), B(1)-C(11) = 1.533(13), B(1)-C(21) = 1.534(13); Br(1)-B(1)-C(11) = 115.2(6), Br(1)B(1)-C(21) = 115.7(6), C(11)-B(1)-C(21) = 129.0(8); Cp(C(11))//Cp(C(21)) = 14.3, Cp(C(11))//Br(1)B(1)C(11)C(21) = 12.2, Cp(C(21))//Br(1)B(1)C(11)C(21) = 12.0; R*(C(11)) = 7.0, R*(C(21)) = 4.3.

lattice of the bromoborane 8 (Table 2) contains two crystallographically independent molecules, 8A and 8B, in the asymmetric unit. Since all key structural parameters of 8A and 8B are very similar within the error margins, only 8A will be considered here (Figure 5). The compound features two cymantrenyl groups and one bromo substituent at a trigonal-planar boron atom. As in the related compounds Fc2BBr (H)33 and Fc#2BOH (4), the bond angle C(11)-B(1)-C(21) = 129.0(8)° is significantly expanded, whereas the two Br(1)-B(1)-C(11)/C(21) angles are con-

tracted to values of about 115°. The B-C and B-Br bond lengths in 8A and H are also largely the same and therefore do not merit further discussion. All three dihedral angles Cp(C(11))//Cp(C(21)), Cp(C(11))//Br(1)B(1)C(11)C(21), and Cp(C(21))//Br(1)B(1)C(11)C(21) in 8A are smaller than 15°, which leads to the conclusion that conformations are possible for this molecule that allow for efficient π conjugation of the cymantrenes via the empty p orbital of the boron bridge. As in the cases of Fc2BBr (H)33 and Fc#2BOH (4), the two metal atoms of 8A are pointing in opposite directions. We note that the dip angles R*(C(11)) = 7.0/R*(C(21)) = 4.3 are less than half the size of those in H (R* = 15.3/11.0),33 thus indicating a smaller degree of Mn 3 3 3 B through-space interaction and in turn a smaller electron density on Mn than on Fe (in addition to electronic factors, the higher steric demand of the Mn(CO)3 fragment in comparison to the FeCp fragment may also play a role). In summary, even though the electron-poor cymantrenyl substituent slows down the Et3SiH-induced condensation reaction to such an extent that (CymBH2)2 ((7)2) becomes an isolable compound, it is nevertheless possible to find reaction conditions that allow for the preparation of Cym2BBr (8). With the cymantrenylborane (CymBH2)2 ((7)2) at our disposal, we next explored whether this compound can be transformed into a stable amine adduct and whether it is a useful hydroboration reagent (Scheme 4). For the synthesis of the amine adduct, excess neat NMe2Et was added at room temperature to solid (7)2. The clear solution was stirred for 30 min and then cooled to -78 °C and slowly concentrated under a dynamic vacuum overnight, whereupon yellow crystals of CymBH2(NMe2Et) formed (10). X-ray crystallography on 10 confirmed the proposed adduct structure (Table 2; Figure 6). Similar to the related ferrocenyl derivative FcBH2(NMe2Et),27 the B-N bond vector in 10 is almost perpendicular to the boron-bound cyclopentadienyl ring (N(1)-B(1)-C(11)-C(12) = 84.8(3)°). In contrast to FcBH2(NMe2Et), however, the bulky ethyl group does not point away from the organometallic substituent but is rotated toward it (C(18)-N(1)-B(1)-C(11) = -69.8(2)°).

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Scheme 4. Synthesis of 10-12a

a Legend: (i) n = 0.5, þ excess NMe2Et, room temperature, 30 min; (ii) n = 1, þ7HCCtBu in THF, room temperature, 5 h; (iii) n = 0.5, þ excess HCCtBu, room temperature, 15 min.

Moreover, the B(1)-N(1) bond is somewhat shorter in 10 (1.633(3) A˚) as compared to FcBH2(NMe2Et) (1.655(7) A˚).27 10 gives rise to a triplet resonance at -5.9 ppm in the 11B NMR spectrum (C6D6; 1JBH = 85 Hz). Thus, the B-N adduct is obviously maintained in solution. Both the chemical shift value and the 1JBH coupling constant are largely the same in 10 and FcBH2(NMe2Et).27 In the 1H NMR spectrum of 10, the BH2 proton signal appears as a broad hump at 2.10-2.90 ppm. The first exploratory studies on the reactivity of (7)2 toward alkynes were carried out in NMR tubes using tertbutylacetylene as reaction partner and THF-d8 as solvent. It soon turned out that the reaction of (7)2 with HCCtBu in stoichiometric ratios of 1:4 or 1:7 results in a mixture of products. We therefore repeated the experiment on a small preparative scale and were able to isolate one of the major species in single crystalline form (i.e., 11; Scheme 4). According to X-ray crystallography (Table 2), the compound is a dinuclear cymantrenyl complex with a B-C-B bridge (Figure 7). The bridging unit bears two (tert-butyl)vinyl side chains (C(31)-C(32) = 1.325(6) A˚, C(41)C(42) = 1.336(6) A˚) and one neopentyl substituent (C(51)C(52) = 1.557(6) A˚). The bond lengths about the trigonalplanar boron atoms vary between B(2)-C(41) = 1.535(6) A˚ and B(1)-C(51) = 1.584(6) A˚. NMR spectroscopy on 11 fully confirms the results of the X-ray crystal structure analysis: in the 1H NMR spectrum, two doublets at 6.40 ppm (2H) and 6.80 ppm (2H) with a coupling constant of 3JHH = 18.0 Hz are indicative of two chemically equivalent (E)-olefinic fragments. A doublet at 2.13 ppm (2H) together with a triplet at 2.88 ppm (1H; 3JHH = 6.1 Hz) can be assigned to the central tBuCH2-CH moiety. A most revealing feature is the pattern of four multiplet signals for the two cyclopentadienyl rings (4.16, 4.19, 4.90, 5.12 ppm; 4  2H). In the H,H-COSY spectrum, each of these resonances shows cross-peaks to at least two of the other resonances. It is thus evident that the observed signal

Figure 6. Molecular structure and numbering scheme of 10. Displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths (A˚), bond angles (deg), and torsion angles (deg): B(1)-N(1) = 1.633(3), B(1)-C(11) = 1.607(3), Mn(1)COav = 1.789(3), C-Oav = 1.155(3), Mn(1) 3 3 3 COG = 1.772; N(1)-B(1)-C(11) = 110.8(2), B(1)-C(11)-C(12) = 128.1(2), B(1)-C(11)-C(15) = 126.6(2); N(1)-B(1)-C(11)-C(12) = 84.8(3), C(18)-N(1)-B(1)-C(11) = -69.8(2); R* = 0.9. COG = centroid of the cyclopentadienyl ring.

pattern is due neither to two different cymantrenyl species nor to two inequivalent cymantrenyl fragments within the same molecule. Instead, all four protons on each cymantrenyl substituent must possess different chemical environments, which can finally be traced back to the presence of a prochiral carbon atom in the B-C-B bridge. The composition of compound 11 provides an interesting insight into the course of the hydroboration reaction, because it shows that a pendant (tert-butyl)vinyl group is able to compete successfully with tert-butylacetylene for the hydroboration reagent. With regard to steric factors this result is contrary to expectation. We therefore speculate that it may have to be explained by electronic peculiarities of the cymantrenylborane. Given this background, (7)2 was next dissolved in a huge excess of neat tert-butylacetylene. A yellow solution formed, which was kept at room temperature for 15 min and then evaporated to dryness in vacuo. NMR spectroscopy of the solid residue in C6D6 gave only one 11B resonance. Its chemical shift value of 56.2 ppm compares well with that of trivinylborane (δ(11B) 56.4).43 The 1H NMR spectrum is characterized by one signal assignable to a tert-butyl group (1.06 ppm), two cyclopentadienyl resonances (4.21, 4.86 ppm), and two signals of (E)-olefins (6.28, 6.75 ppm; 2  d, 3JHH = 17.8 Hz). We therefore confidently propose that the isolated product possesses the structure of 12 (Scheme 4).

Conclusion We have demonstrated that the recently developed condensation reaction 2FcBBr2 þ 3Et3SiH f Fc2BBr þ 0.5B2H6 þ 3Et3SiBr can also be applied to achieve the transformations (43) Hall, L. W.; Odom, J. D.; Ellis, P. D. J. Am. Chem. Soc. 1975, 97, 4527–4531.

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Figure 7. Molecular structure and numbering scheme of 11. Displacement ellipsoids are drawn at the 50% probability level; H atoms are omitted for clarity. Selected bond lengths (A˚), bond angles (deg), and dihedral angles (deg): B(1)-C(11) = 1.563(6), B(1)-C(31) = 1.547(6), B(1)-C(51) = 1.584(6), B(2)-C(21) = 1.567(7), B(2)-C(41) = 1.535(6), B(2)-C(51) = 1.579(6), C(31)C(32) = 1.325(6), C(41)-C(42) = 1.336(6), C(51)-C(52) = 1.557(6); C(11)-B(1)-C(31) = 115.5(4), C(11)-B(1)-C(51) = 122.6(4), C(31)-B(1)-C(51) = 121.9(4), C(21)-B(2)-C(41) = 117.7(4), C(21)-B(2)-C(51) = 119.6(4), C(41)-B(2)-C(51) = 122.3(4), B(1)-C(51)-B(2) = 108.8(3); Cp(C(11))//Cp(C(21)) = 78.0, Cp(C(11))//C(11)B(1)C(31)C(51) = 15.9, Cp(C(21))// C(21)B(2)C(41)C(51) = 21.9, C(11)B(1)C(31)C(51)//C(21)B(2)C(41)C(51) = 79.2; R*(C(11)) = 1.3, R*(C(21)) = 0.7.

Fc#BBr2 f Fc#2BBr and CymBBr2 f Cym2BBr (Fc = (C5H5)Fe(C5H4); Fc# = (C5Me5)Fe(C5H4); Cym = (OC)3Mn(C5H4)). Thus, substantial steric bulk and the use of electron-poor organometallic substituents are both tolerated by this synthetically useful procedure. However, the cymantrenyl substituent slows down the Et3SiH-induced condensation reaction to such an extent that (CymBH2)2 becomes an isolable compound, which is not the case for FcBH2. In a series of preliminary studies, we have further shown that (CymBH2)2 is a potent reagent for the hydroboration of alkynes. With regard to the synthesis of more sophisticated (macromolecular) boron- and transition-metal-containing compounds, these findings are important in two respects. (i) The scope of the condensation reaction, which has already been employed for the preparation of boron-briged poly(ferrocenylene)s [-fcB(X)-]n (X = H, Br; fc = Fe(C5H4)2),27,28,30 can be extended to other organometallic moieties. (ii) The dimeric borane (CymBH2)2 as well as its amine adduct CymBH2(NMe2Et) are promising candidates for the synthesis of organometallic macromolecules via the hydroboration polymerization44-48 of (aromatic) dialkynes. (44) Matsumi, N.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 1998, 120, 5112–5113. (45) Matsumi, N.; Miyata, M.; Chujo, Y. Macromolecules 1999, 32, 4467–4469. (46) Matsumi, N.; Chujo, Y.; Lavastre, O.; Dixneuf, P. H. Organometallics 2001, 20, 2425–2427. (47) Miyata, M.; Chujo, Y. Polym. Bull. 2003, 51, 9–16.

General Remarks. All reactions were carried out under a nitrogen atmosphere using Schlenk tube techniques and carefully dried solvents. NMR: Bruker AM 250, Avance 300, and Avance 400. Chemical shifts are referenced to residual solvent signals (1H, 13C{1H}) or external BF3 3 Et2O (11B{1H}). Abbreviations: s = singlet, d = doublet, tr = triplet, q = quartet, vtr = virtual triplet, br = broad, n.r. = multiplet expected in the 1H NMR spectrum but not resolved, n.o. = signal not observed. BBr3 was stored over mercury under a nitrogen atmosphere. Et3SiH was dried over CaH2 and vacuum-transferred from the drying agent either directly into the reaction flask or into a Schlenk storage vessel. NMe2Et was dried over neat solid n-BuLi (6 h) and then vacuum-transferred to a Schlenk storage vessel. Ferrocene (FcH), cymantrene (CymH), and BBr3 are commercially available; (C5Me5)Fe(C5H5) (Fc#H),49 FcBBr2 (F),32,33,35 and CymBBr2 (6)35,50 can be synthesized according to literature procedures. Synthesis of 2. Compound 1 (0.56 g, 1.32 mmol) was dissolved in toluene (10 mL), and the solution was frozen at -196 °C. Carefully dried Et3SiH (0.46 mL, 0.34 g, 2.90 mmol) was vacuum-transferred into the reaction flask. The vessel was purged with N2 and then warmed to room temperature. The reaction mixture was stirred at room temperature for 48 h and then evaporated to dryness in vacuo. 2 remained in the flask as a red microcrystalline solid residue. Yield: 0.34 g (86%). 1 H NMR (400.1 MHz, C6D6): δ 1.73 (s, 30H; Me), 4.08, 4.43 (n.r., br, 2  4H; C5H4). 11B{1H} NMR (128.4 MHz, C6D6): δ 58.1 (h1/2 = 550 Hz). 13C{1H} NMR (100.6 MHz, C6D6): δ 11.3 (Me), 81.3 (C5Me5), n.o. (C5H4). Synthesis of 3. Neat Me3SiNMe2 (0.22 mL, 0.16 g, 1.37 mmol) was added via syringe at room temperature to a stirred solution of 2 (0.28 g, 0.46 mmol) in toluene (10 mL). The originally red color of the solution changed to orange within 10 min. After 1 h of stirring, all volatiles were driven off under reduced pressure and the product was isolated as an orange-red solid in essentially quantitative yield. 1 H NMR (400.1 MHz, C6D6): δ 1.77 (s, 30H; Me), 3.16 (s, 6H; NMe2), 3.90, 4.27 (2  n.r., 2  4H; C5H4). 11B{1H} NMR (128.4 MHz, C6D6): δ 41.6 (h1/2 = 350 Hz). 13C{1H} NMR (100.6 MHz, C6D6): δ 11.6 (Me), 79.9 (C5Me5), n.o. (NMe2, C5H4). MALDI-MS (norharmane matrix): m/z (%) 565 [M]þ (100). Anal. Calcd for C32H44BFe2N [565.20]: C, 68.00; H, 7.85; N, 2.48. Found: C, 67.52; H, 7.84; N, 2.39. Synthesis of 4. A solution of 3 (0.37 g, 0.65 mmol) in toluene (15 mL) was treated at room temperature with deionized H2O (15 mL). The resulting two-phase system was vigorously stirred for 16 h. The two phases were separated from each other, and the aqueous phase was repeatedly extracted with hexane (3  15 mL). The combined organic phases were evaporated to dryness in vacuo to give 4 as an orange-brown solid. Yield: 0.25 g (72%). Single crystals suitable for X-ray crystallography were grown by slow evaporation of a benzene solution of 4 in air. 1 H NMR (250.1 MHz, C6D6): δ 1.78 (s, 30H; Me), 3.94, 4.09 (2  vtr, 2  4H; C5H4). 11B{1H} NMR (80.3 MHz, C6D6): δ 47.9 (h1/2 = 700 Hz). 13C{1H} NMR (62.9 MHz, C6D6): δ 11.4 (Me), 76.2, 76.6 (C5H4), 80.4 (C5Me5), n.o. (BCipso). MALDIMS (norharmane matrix): m/z (%) 538 [M]þ (100). Anal. Calcd for C30H39BFe2O [538.12] 3 2.5H2O [18.02]: C, 61.79; H, 7.60. Found: C, 61.78; H, 7.65. Synthesis of 5. A solid mixture of F (0.54 g, 1.52 mmol) and 1 (0.65 g, 1.53 mmol) was dissolved in toluene (15 mL), and (48) Lorbach, A.; Bolte, M.; Li, H.; Lerner, H.-W.; Holthausen, M. C.; J€akle, F.; Wagner, M. Angew. Chem., Int. Ed. 2009, 48, 4584– 4588. (49) Bunel, E. E.; Valle, L.; Manriquez, J. M. Organometallics 1985, 4, 1680–1682. (50) Kunz, K.; Vitze, H.; Bolte, M.; Lerner, H.-W.; Wagner, M. Organometallics 2007, 26, 4663–4672.

Article the solution was frozen at -196 °C. Carefully dried Et3SiH (1.08 mL, 0.78 g, 6.74 mmol) was vacuum-transferred into the reaction flask. The vessel was purged with N2 and then warmed to room temperature. After the reaction mixture had been stirred for 30 h at room temperature, all volatiles were driven off in vacuo. The product mixture was obtained as a highly viscous purple oil. 1 H NMR (400.1 MHz, C6D6): δ 1.72 (s, 15H; Me), 4.00 (s, 5H; Cp), 4.06, 4.42, 4.72 (3  n.r., 2H, 4H, 2H; C5H4). 11B{1H} NMR (80.3 MHz, C6D6): δ 56.8 (h1/2 = 600 Hz). 13C{1H} NMR (100.6 MHz, C6D6): δ 11.3 (Me), 70.1 (Cp), 75.0, 77.0, 78.9, 80.0 (C5H4), 81.4 (C5Me5), n.o. (BCipso). Synthesis of (7)2. A solution of 6 (0.75 g, 2.00 mmol) in toluene (2.0 mL) was added with stirring at -78 °C to neat Et3SiH (5.00 mL, 3.64 g, 31.30 mmol). Stirring was continued at -78 °C for 5 min; the precipitate that formed was quickly collected on a frit, washed with hexane (2  10 mL), and dried under reduced pressure. Yield: 0.37 g (86%). Single crystals suitable for an X-ray crystal structure analysis were obtained when the toluene solution of 6 was added without stirring. 1 H NMR (250.1 MHz, THF-d8): δ 4.66, 4.74 (2  vtr, 2  2H; C5H4), n.o. (BH). 11B{1H} NMR (96.3 MHz, THF-d8): δ 4.5 (s, h1/2 = 170 Hz). 11B NMR (96.3 MHz, THF-d8): δ 4.5 (tr, 1JBH = 91 Hz). 13C{1H} NMR (62.9 MHz, THF-d8): δ 84.5, 91.0 (C5H4), n.o. (CO, BCipso). IR (KBr, cm-1): ν~ 2552 (BH), 2013 (CO), 1935 (CO), 1547 (B-H-B). Anal. Calcd for C8H6BMnO3 [215.88]: C, 44.51; H, 2.80. Found: C, 44.95; H, 2.92. Synthesis of 8. Carefully dried neat Et3SiH (0.48 mL, 0.35 g, 3.00 mmol) was added dropwise slowly at room temperature to a stirred solution of 6 (0.74 g, 1.99 mmol) in toluene (15 mL). After the reaction mixture had been stirred for another 20 h, all volatiles were removed in vacuo to give 8 as a pale yellow microcrystalline solid. Yield: 0.48 g (97%). Single crystals suitable for an X-ray crystal structure analysis were obtained by slow evaporation of a toluene solution of 8 under inert conditions. 1 H NMR (250.1 MHz, C6D6): δ 4.09, 4.80 (2  vtr, 2  4H; C5H4). 11B{1H} NMR (80.3 MHz, C6D6): δ 53.2 (h1/2 = 570 Hz). 13C{1H} NMR (62.9 MHz, C6D6): δ 87.0, 94.9 (C5H4), n.o. (CO, BCipso). Synthesis of 9. Neat Me3SiNMe2 (0.34 mL, 0.25 g, 2.13 mmol) was added dropwise with stirring at room temperature to a solution of 8 (0.35 g, 0.71 mmol) in toluene (15 mL). Stirring was continued for 2 h before the solution was evaporated to dryness. The product was obtained as a yellow, microcrystalline solid. Yield: 0.32 g (97%). Single crystals suitable for an X-ray crystal structure analysis were obtained by slow evaporation of a toluene solution of 9 under inert conditions. 1 H NMR (300.1 MHz, C6D6): δ 2.53 (s, 6H; NMe2), 4.15, 4.35 (2  vtr, 2  4H; C5H4). 11B{1H} NMR (96.3 MHz, C6D6): δ 36.7 (h1/2 = 270 Hz). 13C{1H} NMR (62.9 MHz, C6D6): δ 41.9 (NMe2), 84.4, 90.2 (C5H4), n.o. (CO, BCipso). Anal. Calcd for C18H14BMn2NO6 [460.99]: C, 46.90; H, 3.06; N, 3.04. Found: C, 47.17; H, 3.39; N, 2.97. Synthesis of 10. Carefully dried neat NMe2Et (10 mL) was added at room temperature to solid (7)2 (0.45 g, 1.04 mmol). The clear solution was stirred for 30 min and then cooled to -78 °C and slowly concentrated under a dynamic vacuum overnight, whereupon yellow X-ray-quality crystals of 10 formed. Yield: 0.55 g (90%). 1 H NMR (300.0 MHz, C6D6): δ 0.49 (tr, 3H, 3JHH = 7.3 Hz; CH2CH3), 1.64 (s, 6H; Me), 2.06 (q, 2H, 3JHH = 7.3 Hz; CH2CH3), 2.10-2.90 (very br; BH), 4.34, 4.41 (2  vtr, 2  2H; C5H4). 11B{1H} NMR (96.3 MHz, C6D6): δ -5.9 (s, h1/2 = 250 Hz). 11B NMR (96.3 MHz, C6D6): δ -5.9 (tr, 1JBH = 85 Hz). 13C{1H} NMR (75.5 MHz, C6D6): δ 8.1 (CH2CH3), 47.3 (Me), 56.1 (CH2CH3), 84.0, 91.5 (C5H4), 227.0 (CO), n.o. (BCipso). IR (KBr, cm-1): ν~ 2505-2180 (BH),51 2008 (CO), 1914 (CO). Anal. Calcd for C12H17BMnNO3 [289.02]: C, 49.87; H, 5.93; N, 4.84. Found: C, 49.51; H, 5.57; N, 4.35. (51) Carpenter, J. D.; Ault, B. S. J. Phys. Chem. 1991, 95, 3507–3511.

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Hydroboration of tert-Butylacetylene with (7)2. Protocol A. tert-Butylacetylene (1 mL, 0.67 g, 8.12 mmol) was added at room temperature to a stirred solution of (7)2 (0.50 g, 1.16 mmol) in THF (20 mL), and the clear yellow mixture was stirred for 5 h. The solution was cooled to -78 °C and slowly concentrated under a dynamic vacuum overnight, whereupon yellow X-ray-quality crystals of 11 formed. Yield: 0.54 g (69%; note: the crystal crop was contaminated with small amounts of 12). 1 H NMR (300.0 MHz, C6D6): δ 1.05 (s, 18H; Me), 1.10 (s, 9H; Me), 2.13 (d, 2H, 3JHH = 6.1 Hz; CH2-CH), 2.88 (tr, 1H, 3JHH = 6.1 Hz; CH2-CH), 4.16, 4.19, 4.90, 5.12 (4  m, 4  2H; C5H4), 6.40, 6.80 (2  d, 2  2H, 3JHH = 18.0 Hz; CHdCH). 11B{1H} NMR (96.3 MHz, C6D6): δ 64.2 (h1/2 = 550 Hz). 13C{1H} NMR (75.5 MHz, C6D6): δ 28.9, 30.1 (Me), 33.5, 35.5 (CCH3), 38.4 (br; CH2-CH), 45.4 (CH2-CH), 85.7, 86.3, 93.3, 93.9 (C5H4), 162.9 (BCHdCH), n.o. (BCHdCH, CO, BCipso). Protocol B. Solid (7)2 (0.014 g, 0.032 mmol) was placed in an NMR tube, and neat tert-butylacetylene (0.20 mL, 0.134 g, 1.631 mmol) was added, whereupon a yellow solution formed. The reaction is exothermic; a temperature of about 20 °C was maintained by means of a water bath. The solution was kept at 20 °C for 15 min and then evaporated to dryness in vacuo. C6D6 was added, the NMR tube was vacuum-sealed, and the sample was investigated by NMR spectroscopy. One major product (12) was observed (>90%). 1 H NMR (300.0 MHz, C6D6): δ 1.06 (s, 18H; Me), 4.21, 4.86 (2  vtr, 2  2H; C5H4), 6.28, 6.75 (2  d, 2  2H, 3JHH = 17.8 Hz; CHdCH). 11B{1H} NMR (96.3 MHz, C6D6): δ 56.2 (h1/2 = 750 Hz). 13C{1H} NMR (75.5 MHz, C6D6): δ 29.1 (Me), 35.4 (CCH3), 86.3, 94.2 (C5H4), 165.9 (BCHdCH), 224.5 (CO), n.o. (BCHdCH, BCipso). IR (KBr, cm-1): ν~ 2024 (CO), 1935 (CO). X-ray Crystal Structure Analysis of 1, 4 3 H2O, (7)2, and 8-11. Data were collected on a STOE IPDS II two-circle diffractometer with graphite-monochromated Mo KR radiation. Empirical absorption corrections were performed for all structures using the MULABS52 option in PLATON.53 The structures were solved by direct methods using the program SHELXS54 and refined against F2 with full-matrix least-squares techniques using the program SHELXL-97.55 The coordinates of H atoms bonded to O atoms in 4 3 H2O were refined with the O-H distances restrained to 0.84(1) A˚. The H atoms bonded to B in (7)2 were refined isotropically. In 11, a terminal tert-butyl group is disordered over two positions with a site occupation factor of 0.53(2) for the major occupied site. The disordered atoms were refined isotropically, and the bond lengths and angles involving the disordered atoms were restrained to be equal to those of an ordered tert-butyl group. CCDC reference numbers: 776061 (1), 776062 (4 3 H2O), 776063 ((7)2), 776065 (8), 776066 (9), 776064 (10), and 776067 (11).

Acknowledgment. M.W. gratefully acknowledges financial support by the Beilstein Institut, Frankfurt/Main, Germany, within the research collaboration NanoBiC. We wish to thank Prof. Dr. A. Terfort for a generous donation of (pentamethyl)ferrocene. Supporting Information Available: CIF files giving crystallographic data of 1, 4 3 H2O, (7)2, and 8-11, text giving details of the synthesis and NMR spectroscopic characterization of Fc#BBr2 (1) and Fc#B(NMe2)2, a figure giving a plot of the dimeric aggregates of 4 3 H2O in the solid state, and a figure and a table giving details of the X-ray crystal structure analysis of Cym2BNMe2 (9). This material is available free of charge via the Internet at http://pubs.acs.org. (52) Blessing, R. H. Acta Crystallogr. 1995, A51, 33–38. (53) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13. (54) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473. (55) Sheldrick, G. M. SHELXL-97: A Program for the Refinement of Crystal Structures; Universit€at G€ottingen, G€ottingen, Germany, 1997.