Half-Sandwich Group 8 Borylene Complexes ... - ACS Publications

Organometallics 2009, 28, 2947–2960

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Half-Sandwich Group 8 Borylene Complexes: Synthetic and Structural Studies and Oxygen Atom Abstraction Chemistry Glesni A. Pierce, Dragoslav Vidovic, Deborah L. Kays, Natalie D. Coombs, Amber L. Thompson, Eluvathingal D. Jemmis,‡,§ Susmita De,‡ and Simon Aldridge* Inorganic Chemistry, UniVersity of Oxford, South Parks Road, Oxford, UK OX1 3QR ReceiVed December 23, 2008

Cationic terminal borylene complexes, synthesized by halide abstraction, offer a versatile platform on which to gauge the effects on the electronic structure of the metal-ligand bond brought about by variation in the borylene substituent and the metal/ligand framework. While the borylene substituent exerts a strong influence on boron-centered electrophilicity and hence on metal-ligand π character and bond length (e.g., from 1.792(8) Å for [Cp*Fe(CO)2(BMes)]+ to 2.049(4) Å for [CpFe(CO)2{B(NCy2)(4pic)}]+), much smaller changes are effected by changes in the metal/ligand set. Introduction of stronger π donor ruthenium- and/or phosphine-containing fragments is readily brought about by extension of the halide abstraction approach; phosphines are readily introduced by carbonyl ligand substitution at the boryl precursor stage. Thus, the novel systems [CpRu(CO)2{B(NCy2)}]+[BAr f4]-, [CpM(CO)(PMe3){B(NCy2)}]+[BAr f4]- (M ) Fe, Ru), and [(CpFe(CO){B(NCy2)})2(µ-dmpe)]2+[BAr f4]-2 have been synthesized and structurally characterized. The effects of substitution at the metal on the M-B and B-N bonds are relatively minor {e.g., Fe-B and B-N bond lengths of 1.821(4), 1.347(5) Å and 1.859(6), 1.324(7) Å for [CpFe(CO)(PMe3){B(NCy2)}]+[BAr f4]- and [CpFe(CO)2{B(NCy2)}]+[BAr f4]-, respectively}, presumably reflecting, at least in part, the mutually cis disposition of the borylene and phosphine/carbonyl ligands. The utility of cationic complexes containing formally subvalent boron-based ligands in oxygen atom abstraction chemistry has been demonstrated by the conversion of a range of isocyanates, R′NCO, to the corresponding (metalcoordinated) isonitriles, [CpFe(CO)2(CNR′)]+. Moreover, with a view to modeling potential intermediates, the mechanism of related chemistry with carbodiimide substrates, R′NCNR′, has been investigated by structural and in situ ESI-MS approaches. While reactivity toward carbodiimides leads to the formation of a novel spirocyclic boronium complex, [CpFe(CO)2C(NCy)2B(NCy)2CNCy2]+[BAr f4]- (for R′ ) Cy), by a double-insertion process, DFT studies imply that the analogous product of isocyanate insertion is unlikely to be an intermediate on the pathway to isonitrile formation. The presence of a number of facile competing reaction pathways (including metathesis) and the thermodynamic stability of the spirocyclic product mean that the product distribution is better explained in terms of competing pathways, rather than differing extents of reaction along similar trajectories. Introduction The predominant routes to transition metal borylene complexes [(LnM)mBX, m ) 1, 2, 3], namely, salt elimination and photolytic transfer chemistries, have been instrumental in accessing two coordinate systems containing MdB double bonds.1 Such compounds have excited interest not only from a desire to relate fundamental issues of geometric and electronic structure to patterns of chemical reactivity but also due to obvious parallels in the classical organometallic * Corresponding author. E-mail: [email protected]. ‡ Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India 560012. § School of Chemical Sciences, Indian Institute of Science Education and Research Thiruvananthapuram, CET Campus, Thiruvananthapuram 695016, Kerala, India. (1) For recent reviews of borylene chemistry see: (a) Braunschweig, H. AdV. Organomet. Chem. 2004, 51, 163. (b) Aldridge, S.; Coombs, D. L. Coord. Chem. ReV. 2004, 248, 535. (c) Braunschweig, H.; Rais, D. Heteroat. Chem. 2005, 16, 566. (d) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254. (e) Aldridge, S.; Kays, D. L. Main Group Chem. 2006, 5, 223. (f) Braunschweig, H.; Kollann, C.; Seeler, F. Struct. Bonding (Berlin) 2008, 130, 1.

chemistry of carbonyl, carbene, and vinylidene ligands.2 Utilizing these synthetic approaches, structurally authenticated species of the type LnMdBX have been reported featuring metals from groups 5, 6, 8, and 9.3 Mirroring chemistry reported for unsaturated group 14 ligand systems,4 we have sought to exploit halide abstraction chemistry to access cationic diyl systems, [LnM(EX)]+ (E ) group 13 (2) See, for example: Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Sausalito, CA, 1987. (3) Salt elimination and intermetallic borylene transfer: (a) Cowley, A. H.; Lomelı´, V.; Voigt, A. J. Am. Chem. Soc. 1998, 120, 6401. (b) Braunschweig, H.; Kollann, C.; Englert, U. Angew. Chem., Int. Ed. 1998, 37, 3179. (c) Braunschweig, H.; Colling, M.; Kollann, C.; Stammler, H. G.; Neumann, B. Angew. Chem., Int. Ed. 2001, 40, 2298. (d) Braunschweig, H.; Colling, M.; Kollann, C.; Merz, K.; Radacki, K. Angew. Chem., Int. Ed. 2001, 40, 4198. (e) Braunschweig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2003, 42, 205. (f) Braunschweig, H.; Radacki, K.; Scheschkewitz, D.; Whittell, G. R. Angew. Chem., Int. Ed. 2005, 44, 1658. (g) Braunschweig, H.; Whittell, G. R. Chem.sEur. J. 2005, 11, 6128. (h) Blank, B.; Colling-Hendelkens, M.; Kollann, C.; Radacki, K.; Rais, D.; Uttinger, K.; Whittell, G. R.; Braunschweig, H. Chem.sEur. J. 2007, 13, 4770. (i) Braunschweig, H.; Forster, M.; Kupfer, T.; Seeler, F. Angew. Chem., Int. Ed. 2008, 47, 5981.

10.1021/om801215b CCC: $40.75  2009 American Chemical Society Publication on Web 04/23/2009

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element), not only to broaden the range of available synthetic routes but also to assess the influence of overall charge on the chemistry of these complexes.5,6 The success of this strategy in giving access to cationic two-coordinate borylene complexes {e.g., [Cp*Fe(CO)2(BMes)]+[BAr f4]- (Mes ) C6H2Me3-2,4,6; Ar f ) C6H3(CF3)2-3,5)} and to related systems containing the heavier group 13 elements as donor atoms {e.g., [Cp*Fe(dppe)(GaI)]+[BAr f4]-} has recently been demonstrated.7 Recent reports (i) of the implication of borylene systems in catalytic processes8 and (ii) of the direct synthesis of borylene complexes from boron dihydride reagents6l have given added impetus to studies mapping the fundamental reactivity patterns of borylene complexes toward key organic substrates. With this in mind, and given the well-known enhancements in complex stability that can be brought about, for example, in related carbene systems, by appropriate choice of metal ancillary ligand and carbene substituent {cf. [CpFe(CO)2(CH2)]+ vs [CpFe(dppe)(CH2)]+ or [CpFe(CO)2{C(OMe)Me}]+},9 we have sought to investigate the scope of variation in electronic structure (and by implication complex stability/reactivity) that can be brought about through changes in the metal/ligand fragment LnM and the borylene substituent X. Thus, we report in-depth synthetic and structual studies of cationic terminal borylenes designed to address this point, together with an investigation of the use of one such complex in oxygen atom abstraction (4) See, for example: (a) Grumbine, S. K.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J. Am. Chem. Soc. 1994, 116, 5495. For a discussion of the use of related electron-rich metal/ligand frameworks in stabilizing dihalocarbene ligands see, for example: (b) Brothers, P. J.; Roper, W. R. Chem. ReV. 1988, 88, 1293. (5) Halide abstraction: (a) Coombs, D. L.; Aldridge, S.; Jones, C.; Willock, D. J. J. Am. Chem. Soc. 2003, 125, 6356. (b) Coombs, D. L.; Aldridge, S.; Rossin, A.; Jones, C.; Willock, D. J. Organometallics 2004, 23, 2911. (c) Kays, D. L.; Aldridge, S.; Day, J. K.; Ooi, L.-L. Angew. Chem., Int Ed. 2005, 44, 7457. (d) Kays, D. L.; Rossin, A.; Day, J. K.; Ooi, L.-L.; Aldridge, S. Dalton. Trans. 2006, 399. (e) Aldridge, S.; Jones, C.; GansEichler, T.; Stasch, A.; Kays, D. L.; Coombs, N. D.; Willock, D. J. Angew. Chem., Int. Ed. 2006, 45, 6118. (f) Vidovic, D.; Findlater, M.; Reeske, G.; Cowley, A. H. Chem. Commun. 2006, 3786. (g) Braunschweig, H.; Radacki, K.; Uttinger, K. Angew. Chem., Int. Ed. 2007, 46, 3979. (h) Braunschweig, H.; Kraft, K.; Kupfer, T.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2008, 47, 4931. (6) Other routes to borylene systems: (a) Irvine, G. J.; Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Angew. Chem., Int. Ed. 2000, 39, 948. (b) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Organometallics 2002, 21, 4862. (c) Braunschweig, H.; Radacki, K.; Rais, D.; Scheschkewitz, D. Angew. Chem., Int. Ed. 2005, 44, 5651. (d) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Organometallics 2006, 25, 5159. (e) Kays, D. L.; Day, J. K.; Aldridge, S.; Harrington, R. W.; Clegg, W. Angew. Chem., Int. Ed. 2006, 45, 3513. (f) Braunschweig, H.; Radacki, K.; Rais, D.; Seeler, F. Angew. Chem., Int. Ed. 2006, 45, 1066. (g) Braunschweig, H.; Radacki, K.; Rais, D.; Uttinger, K. Angew. Chem., Int. Ed. 2006, 45, 162. (h) Braunschweig, H.; Radacki, K.; Rais, D.; Schneider, A.; Seeler, F. J. Am. Chem. Soc. 2007, 129, 10350. (i) Braunschweig, H.; Kupfer, T.; Radacki, K.; Schneider, A.; Seeler, F.; Uttinger, K.; Wu, H. J. Am. Chem. Soc. 2008, 130, 7974. (j) Braunschweig, H.; Burzler, M.; Kupfer, T.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 7785. (k) Braunschweig, H.; Burzler, M.; Dewhurst, R. D.; Radacki, K. Angew. Chem., Int. Ed. 2008, 47, 5650. (l) Alcaraz, G.; Helmstedt, U.; Clot, E.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2008, 130, 12878. (7) (a) Coombs, N. D.; Clegg, W.; Thompson, A. L.; Willock, D. J.; Aldridge, S. J. Am. Chem. Soc. 2008, 130, 5449. (b) Himmel, H.-J.; Linti, G. Angew. Chem., Int. Ed. 2008, 47, 6326. (c) Coombs, N. D.; Vidovic, D.; Day, J. K.; Thompson, A. L.; Le Pevelen, D. D.; Stasch, A.; Clegg, W.; Russo, W.; Male, L.; Hursthouse, M. B.; Willock, D. J.; Aldridge, S. J. Am. Chem. Soc. 2008, 130, 16111. (8) Apostolico, L.; Braunschweig, H.; Crawford, A. G.; Herbst, T.; Rais, D. Chem. Commun. 2008, 497. (9) (a) Studabaker, W. B.; Brookhart, M. J. Organomet. Chem. 1986, 310, 39. (b) Bodnar, T.; LaCroce, S. J.; Cutler, A. R. J. Am. Chem. Soc. 1980, 102, 3292.

Pierce et al.

chemistry by crystallographic, NMR, in situ ESI-MS, and quantum chemical methods.10

Experimental Section (i) General Considerations. All manipulations were carried out under a nitrogen or argon atmosphere using standard Schlenk line or drybox techniques. Solvents (dichloromethane, diethyl ether, toluene, hexanes, pentane) were dried using a commercial Braun SPS system and stored over molecular sieves before use; fluorobenzene was distilled from CaH2. d6-Benzene, d8-toluene, and d2-dichloromethane (all Goss) were degassed and dried over the appropriate drying agent (potassium or molecular sieves) prior to use. Reagents PMe3, dmpe, 13CO2, and dicyclohexylcarbodiimide were used as received; 4-picoline (sodium) and R′NCO (R′ ) Cy, Ph, 2,6-Xyl) (molecular sieves) were dried over the appropriate drying agent before use. Na[BAr f4], Na[(η5C5H5)Ru(CO)2], and Cy2NBCl2 were prepared by literature methods.11 Outline data for compounds CpFe(CO)2{B(NCy2)Cl} (1), [CpFe(CO)2{B(NCy2)}]+[BAr f4]- (3), and [CpFe(CO)2C(NCy)2BNR2]+[BAr f4]- (16) have previously been included in a preliminary communication;10 full synthetic, spectroscopic, and crystallographic details are included only in the Supporting Information. Likewise, selected data for [CpFe(CO)(PMe3)2]+Clhave been reported previously;12 complete spectroscopic data are included in the Supporting Information. NMR spectra were measured on a Bruker AM-400 or Jeol 300 Eclipse Plus FT-NMR spectrometer. Residual signals of solvent were used for reference for 1H and 13C NMR spectroscopy. 11B, 19 F, and 31P NMR spectra were referenced with respect to Et2O · BF3, CFCl3, and 85% aqueous H3PO4, respectively. Infrared spectra were measured for each compound either pressed into a disk with excess dry KBr or as a solution in the appropriate solvent, on a Nicolet 500 FT-IR spectrometer. Mass spectra were measured on a Bruker MicroTOF instrument with direct sampling from a Braun LabMaster inert atmosphere box at the University of Bath (cationic complexes) or by the EPSRC National Mass Spectrometry Service Centre, at the University of Wales Swansea (neutral precursors). Perfluorotributylamine was used as the standard for high-resolution EI mass spectra. In situ monitoring of reactions by ESI-MS was carried out using the methods reported by Lubben et al.13 Photolytic experiments were carried out using a Spectral Energy mercury arc lamp (1 kW) with samples contained within quartz Schlenk vessels. Characterization of new compounds is based on multinuclear NMR, IR, and mass spectrometry data (including accurate mass measurement), supplemented by elemental microanalysis and by singlecrystal X-ray diffraction studies in the cases of 1-6, 9, 10, 13, 16, 17, and 18b. In the cases of compounds 4, 6, and 13, reliable microanalyses proved impossible to obtain, possibly due to the presence of volatile solvent in the lattice; in all cases the purity of the bulk material was established by multinuclear NMR to be >95%. Abbreviations: br ) broad, s ) singlet, d ) doublet, q ) quartet, m ) multiplet, fwhm ) frequency width at half-maximum. (ii) Crystallographic and Computational Methods. Data for compounds 1, 2, 3 · CH2Cl2, 4 · CH2Cl2, 5, 6 · 1/2C5H12, 9 · CH2Cl2, 10, 13 · 1/2CH2Cl2, 16 · 1/2C6H14, 17, and 18b · 1/2C6H5F were (10) Preliminary accounts of part of this work have been communicated: see ref 5e and: Pierce, G. A.; Aldridge, S.; Jones, C.; Gans-Eichler, T.; Stasch, A.; Coombs, N. D.; Willock, D. J. Angew. Chem., Int. Ed. 2007, 46, 2043. (11) (a) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith, M. D. Inorg. Chem. 2001, 40, 3810. (b) King, R. B. Acc. Chem. Res. 1970, 3, 417. (c) Maringgele, W.; Noltemeyer, M.; Meller, A. Organometallics 1997, 16, 2276. (d) Braunschweig, H.; Kollann, C.; Englert, U. Eur. J. Inorg. Chem. 1998, 465. (12) Bellinger, G. C. A.; Friedrich, H. B.; Moss, J. R. J. Organomet. Chem. 1989, 366, 175. (13) Lubben, A. T.; McIndoe, J. S.; Weller, A. S. Organometallics 2008, 27, 3303.

Half-Sandwich Group 8 Borylene Complexes collected at low temperature using an Oxford Cryosystems Cryostream N2 open-flow cooling device on an Enraf Nonius Kappa CCD diffractometer.14a Data collection and cell refinement were carried out using DENZO and COLLECT, and structure solution and refinement using CRYSTALS, SIR-92, SHELXS-97, and SHELXL-97; absorption corrections were performed using SORTAV.14b-g With the exception of 1, 3 · CH2Cl2, 16 · 1/2C6H14, and 17 (previously included in a Communication), which are included in the Supporting Information only, the details of each data collection, structure solution, and refinement can be found in Table 1. Relevant bond lengths and angles are included in the figure captions, and complete details of each structure have been deposited with the CCDC. In addition, CIFs for each structure have been included in the Supporting Information. In general, all non-hydrogen atoms were refined with anisotropic displacement parameters. Exceptions to this were where there was disorder, for example as seen in CF3 groups in compound 10, where a partially isotropic model was used to describe the thermal motion. Hydrogen atoms were either initially found in the difference map and refined with soft constraints or added geometrically and then included in the model with the parameters riding on those of the parent atom. In the case of compound 13, examination of the difference map indicated the presence of diffuse electron density. Efforts made to model it were unsuccessful, so SQUEEZE within PLATON14h,i was used to provide the discrete Fourier transform of the void region, which was treated as contributions to the A and B parts of the calculated structure factors within CRYSTALS. The computational approaches utilized both for geometry optimization processes and for the calculation of σ and π contributions to bonding densities were as reported previously for analogous investigations of transition metal diyl and boryl complexes.15 Full details of the methodology used for the quantum chemical mechanistic studies are included in an associated article.16 (iii) Syntheses. CpRu(CO)2{B(NCy2)Cl} (2). A mixture of Cy2NBCl2 (1.13 g, 4.31 mmol), Na[CpRu(CO)2] (1.1 equiv), and diethyl ether (30 cm3) was stirred at 20 °C for 12 h. After removal of solvent under reduced pressure, the resulting solid was extracted with pentane (50 cm3). Concentration to ca. 10 cm3 and storage -30 °C led to the isolation of 2 as pale yellow crystals suitable for X-ray diffraction. A further crop of crystalline material could be obtained by concentrating the supernatant solution to ca. 3 cm3 and storage at -30 °C. Isolated yield: 1.32 g, 68%. 1H NMR (300 MHz, C6D6): δ 1.15-1.70 (overlapping m, 20H, CH2 of Cy), 2.86 (br m, 2H, CH of Cy), 4.71 (s, 5H, Cp). 13C NMR (75 MHz, C6D6): δ 25.7, 25.8, 26.3, 27.5, 32.4, 33.6 (CH2 of Cy), 58.9, 66.0 (CH of Cy), 87.4 (Cp), 201.9 (CO). 11B NMR (96 MHz, C6D6): δ 50. IR (C6D6 soln, cm-1): ν(CO) 1990, 1928. MS(EI): m/z (%) 442 (2) M+; exact mass (calc for M+) 442.0930, (measd) 442.0936. [CpRu(CO)2{B(NCy2)}]+[BAr f4]- (4). A mixture of 2 (0.10 g, 0.22 mmol) and Na[BAr f4] (1.1 equiv) in dichloromethane (20 cm3) (14) (a) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105. (b) Denzo: Otwinowski, Z.; Minor, W. Methods in Enzymology; Carter, C. W., Sweet, R. M., Eds.; Academic Press: New York, 1996; Vol. 276, p 307. (c) Collect:, Nonius, B. V.: Delft, The Netherlands, 1997-2002. (d) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. (e) Sir-92: Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (f) CRYSTALS: Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, J.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487. (g) Sortav: Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33. (h) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7. (i) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht, The Netherlands, 1998. (15) For previous DFT studies of related systems see, for example: (a) Dickinson, A. A.; Willock, D. J.; Calder, R. J.; Aldridge, S. Organometallics 2002, 21, 1146. (b) Aldridge, S.; Rossin, A.; Coombs, D. L.; Willock, D. J. Dalton Trans. 2004, 2649. (16) For a complementary DFT study of insertion and metathesis reactivity involving carbodiimide and isocyanate substrates see: De, S.; Pierce, G. A.; Vidovic, D.; Kays, D. L.; Coombs, N. D.; Jemmis, E. D.; Aldridge, S. Organometallics, http:// dx.doi.org/10.1021/om801214f.

Organometallics, Vol. 28, No. 10, 2009 2949 was stirred for 12 h, after which the reaction was judged to be complete by 11B NMR spectroscopy. Filtration of the mixture, concentration of the filtrate to ca. 5 cm3, layering with hexanes (ca. 10 cm3), and storage at -30 °C led to the formation of 4 as pale yellow crystals (of the dichloromethane solvate) suitable for X-ray diffraction. Isolated yield: 0.18 g, 63%. 1H NMR (300 MHz, CD2Cl2): δ 1.09-2.11 (overlapping m, 20H, CH2 of Cy), 2.88 (m, 2H, CH of Cy), 5.61 (s, 5H, Cp), 7.49 (s, 4H, para-CH of BAr f4-), 7.64 (s, 8H, ortho-CH of BAr f4-). 13C NMR (75 MHz, CD2Cl2): δ 25.1, 26.0 36.6 (CH2 of Cy), 59.3 (CH of Cy), 90.0 (Cp), 117.8 (para-CH of BAr f4-), 124.8 (q, 1JCF ) 272 Hz, CF3 of BAr f4-), 129.0 (q, 2JCF ) 31 Hz, meta-C of BAr f4-), 135.0 (ortho-CH of BAr f4-), 161.9 (q, 1JCB ) 50 Hz, ipso-C of BAr f4-), 192.8 (CO). 11 B NMR (96 MHz, CD2Cl2): δ -7.6 (BAr f4-), 90 (br, fwhm ca. 860 Hz, borylene). 19F NMR (283 MHz, CD2Cl2): δ -62.7 (CF3). IR (CD2Cl2 soln, cm-1): ν(CO) 2022, 1957. MS (positive ion electrospray): m/z (%) 414 (100) M+; exact mass (calc for M+) 414.1187, (measd) 414.1181. [CpFe(CO)2{B(NCy2)(4-pic)}]+[BAr f4]- (5). A mixture of 3 (0.18 g, 0.18 mmol) and 4-picoline (5.0 equiv) in dichloromethane (5 cm3) was stirred at room temperature for 1 h. Concentration to ca. 2 cm3, layering with hexanes (ca. 10 cm3), and storage at -30 °C led to the formation of 5 as colorless crystals suitable for X-ray diffraction. Isolated yield: 0.10 g, 52%. 1H NMR (400 MHz, CD2Cl2): δ 1.00-1.75 (m, 20H, CH2 of Cy), 2.44 (s, 3H, CH3 of 4-pic), 2.90 (m, 1H, CH of Cy), 3.91 (m, 1H, CH of Cy), 4.71 (s, 5H, Cp), 7.49 (s, 4H, para-CH of BAr f4-), 7.54 (d, 3J ) 6.3 Hz, 2H, CH of 4-pic), 7.64 (s, 8H, ortho-CH of BAr f4-), 7.94 (d 3J ) 6.3 Hz, 2H, CH of 4-pic). 13C NMR (75 MHz, CD2Cl2): δ 22.4 (CH3 of 4-pic), 25.5, 25.6, 26.4, 27.3, 36.6, 37.7 (CH2 of Cy), 60.4, 66.2 (CH of Cy), 84.7 (Cp), 116.8 (para-CH of BAr f4-), 123.8 (q, 1 JCF ) 272 Hz, CF3 of BAr f4-), 128.1 (aromatic CH of 4-pic), 128.4 (q, 2JCF ) 31 Hz, meta-C of BAr f4-), 134.1 (ortho-CH of BAr f4-), 141.5 (aromatic CH of 4-pic), 158.2 (quaternary of 4-pic), 161.0 (q, 1JCB ) 50 Hz, ipso-C of BArf4-), 213.0 (CO). 11B NMR (96 MHz, CD2Cl2): δ -7.6 (BAr f4-), 57 (br, fwhm ca. 320 Hz, borylene). 19F NMR (283 MHz, CD2Cl2): δ -62.7 (CF3). IR (CD2Cl2 soln, cm-1): ν(CO) 2019, 1962. Anal. (calc for 5) C 51.69, H 3.50, N 2.11; (measd) C 51.62, H 3.53, N 2.42. CpFe(CO)(PMe3){B(NCy2)Cl} (6), CpRu(CO)(PMe3){B(NCy2)Cl} (7), and CpFe(CO)(PPh3){B(NCy2)Cl} (8). A common method was employed for all three compounds, exemplified for 6. A mixture of CpFe(CO)2{B(NCy2)Cl} (1; 0.20 g, 0.5 mmol) and PMe3 (2 equiv) in toluene was photolyzed for 6 h with stirring, after which the reaction was judged to be complete by 11B and 31P NMR spectroscopy. After the removal of volatiles under reduced pressure and extraction into pentane (ca. 30 cm3), storage at -80 °C led to the formation of 6 as red crystals (of the pentane hemisolvate) suitable for X-ray diffraction. Yield: 0.07 g, 33%. 1H NMR (300 MHz, C6D6): δ 0.93-2.18 (overlapping m, 20H, CH2 of Cy), 1.14 (d, 2JHP ) 9 Hz, 9H, PMe3), 2.10, 2.97 (m, each 2H, CH of Cy), 4.26 (s, 5H, Cp). 13C NMR (126 MHz, C6D6): δ 20.8 (d, 1JCP ) 30 Hz, PMe3), 26.5, 26.6, 26.7, 26.8, 28.1 28.2, 33.1, 33.9, 34.6, 34.7 (CH2 of NCy2), 59.2, 65.1 (CH of NCy2), 82.2 (Cp), 220 2 (d, 2JCP ) 38 Hz, CO). 11B NMR (96 MHz, C6D6): δ 62 (br, fwhm 500 Hz). 31P NMR (122 MHz, C6D6): δ 41.5. IR (CD2Cl2 soln, cm-1) ν 1890 (CO). MS(EI): m/z (%) 451 (4) (M)+, 423.1 (19) (M - CO)+, 389.2 (25) (M - CO - Cl)+; exact mass (calcd for M+, 10B isotopomer) m/z 450.1696, (measd) 450.1697. 7 was prepared in a similar manner from 2 and PMe3 and isolated as a pale yellow oil in yields of ca. 36%. Data for 7: 1H NMR (300 MHz, C6D6): δ 1.02-2.08 (overlapping m, 20H, CH2 of Cy), 1.16 (d, JHP ) 9 Hz, 9H, PMe3), 2.94 (m, 2H, CH of Cy), 4.82 (s, 5H, Cp). 13C NMR (126 MHz, C6D6): δ 20.6 (d, 1JCP ) 33 Hz, PMe3), 24.7, 24.8, 25.6, 25.8, 26.3, 26.4, 31.4, 32.0, 32.7, 33.1 (CH2 of NCy2), 57.6, 65.0 (CH of NCy2), 85.5 (3JCP ) 2 Hz, Cp), 208.7 (d, 2 JCP ) 24 Hz, CO). 11B NMR (96 MHz, C6D6): δ 57 (br, fwhm


Pierce et al.

Table 1. Crystallographic data for 2, 4 · CH2Cl2, 5, 6 · 1/2C5H12, 9 · CH2Cl2, 10, 13 · 1/2CH2Cl2, and 18b · 1/2C6H5F CCDC deposition number empirical formula fw temperature (K) wavelength (Å) cryst syst space group unit cell lengths: a, b, c (Å) R, β, γ (deg) volume (Å3), Z density(calc) (Mg/m3) absorp coeff (mm-1) F(000) cryst size (mm3) θ range for data collectn (deg) index ranges (h, k, l) no. of reflns collected no. of indep reflns/Rint completeness to θmax (%) absorp corr max. and min. transmn refinement method no. data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) Largest peak/hole (e Å-3)

CCDC deposition number empirical formula fw temperature (K) wavelength (Å) cryst syst space group unit cell lengths: a, b, c (Å) R, β, γ (deg) volume (Å3), Z density(calc) (Mg/m3) absorp coeff (mm-1) F(000) cryst size (mm3) θ range for data collectn (deg) index ranges (h, k, l) no. of reflns collected no. of indep reflns/Rint completeness to θmax (%) absorp corr max. and min. transmn refinement method no. data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest peak/hole (e Å-3)

2

4 · CH2Cl2

5

706205 C19H27B1Cl1N1O2Ru1 448.76 150 0.71073 monoclinic P1 21/c 1 11.8422(2) 11.8546(2) 14.4183(2) 90, 98.7952(7), 90 2000.31(6), 4 1.490 0.929 920 0.43 × 0.39 × 0.20 5.13 to 27.49 -15 to 15, -15 to 15, -18 to 18 27 804 4565, 0.033 99.3 semiempirical from equivs 0.83 and 0.74

706206 C52H41B2Cl2F24N1O2Ru1 1362.45 150 0.71073 triclinic P1j 12.7397(2) 14.7060(3) 16.4103(3) 77.167(1), 74.081(1), 74.425(1) 2811.17(9), 2 1.608 0.495 1360 0.25 × 0.25 × 0.16 5.12 to 27.48 -16 to 16, -19 to 19, -21 to 19 41 940 12 755, 0.050 99.1 semiempirical from equivs 0.92 and 0.84 full matrix least-squares on F2 7665/12/754 1.138 R1 ) 0.0769, wR2 ) 0.0774 R1 ) 0.1233, wR2 ) 0.1068 1.15 and -0.81

609045 C57H46B2F24FeN2O2 1324.43 150 0.71073 triclinic P1j 12.512(3) 14.644(3) 16.427(3) 88.47(3), 81.94(3), 86.92(3) 2975.3(10), 2 1.478 0.372 1340 0.25 × 0.20 × 0.08 3.03 to 24.99 -14 to 14, -17 to 17, -19 to 19 18 669 10 322, 0.0309 98.5 Sortav 0.97 and 0.81

9 · CH2Cl2 706208 C54H50B2Cl2F24Fe1N1O1P1 1364.30 150 0.71073 monoclinic C 1 2/c 1 17.4810(1) 18.2783(1) 37.2971(3) 90, 96.0302(3), 90 11851.34(13), 8 1.529 0.487 5520 0.30 × 0.26 × 0.24 5.10 to 27.50 -22 to 22, -23 to 23, -48 to 48 63 388 13 462, 0.053 99.0 semiempirical from equivs 0.89 andd 0.84 full matrix least-squares on F2 13 461/12/787 0.995 R1 ) 0.0636, wR2 ) 0.1516 R1 ) 0.1025, wR2 ) 0.1711 0.92 and -0.70

10 706209 C53H48B2F24N1O1P1Ru1 1324.59 150 0.71073 monoclinic P 1 21/c 1 12.2390(1) 17.1260(2) 27.0465(3) 90, 99.1154(4), 90 5597.49(10), 4 1.572 0.428 2664 0.25 × 0.20 × 0.15 5.13 to 27.48 -15 to 15, -22 to 22, -35 to 35 50 002 12 677, 0.042 99.0

4565/0/226 0.986 R1 ) 0.0260, wR2 ) 0.0636 R1 ) 0.0312, wR2 ) 0.0648 0.68 and -0.71

6 · 1/2C5H12 706207 C23.5H42B1Cl1Fe1N1O1P1 487.68 150 0.71073 monoclinic P 1 21/n 1 13.7562(1) 12.3020(1) 15.9619(1) 90, 93.8710(4), 90 2695.05(3), 4 1.202 0.732 1044 0.25 × 0.18 × 0.15 5.12 to 27.50 -17 to 17, -15 to 15, -20 to 20 51 592 6134, 0.039 99.3 0.90 and 0.84 6134/24/254 0.965 R1 ) 0.0430, wR2 ) 0.1068 R1 ) 0.0541, wR2 ) 0.1123 0.61 and -0.49

CCDC deposition number empirical formula fw temperature (K) wavelength (Å) cryst syst space group unit cell lengths: a, b, c (Å) R, β, γ (deg) volume (Å3), Z density(calc) (Mg/m3) absorp coeff (mm-1) F(000) cryst size (mm3) θ range for data collectn (deg)

13 1/2CH2Cl2 706210 C53.5H48B2Cl1F24Fe1N1O1P1 1380.82 150 0.71073 triclinic P1j 12.6073(1) 19.6463(2) 26.4427(4) 70.530(1), 87.053(1), 78.537(1) 6050.80(12) 1.450 0.431 2672 0.22 × 0.16 × 0.12 5.10 to 27.45

10 322/30/795 1.037 R1 ) 0.0605, wR2 ) 0.1430 R1 ) 0.0799, wR2 ) 0.1539 1.06 and -0.64

0.94 and 0.85 9103/168/800 1.162 R1 ) 0.0584, wR2 ) 0.0587 R1 ) 0.0910, wR2 ) 0.0782 1.88 and -0.93

18b 1/2C6H5F 706211 C49H24.5BF24.5FeNO2 1191.36 150 0.71073 orthorhombic Pbca 17.9155(2) 16.2092(3) 33.7381(5) 90, 90, 90 9797.4(3), 8 1.615 0.443 4744 0.55 × 0.44 × 0.23 5.12 to 27.50

Half-Sandwich Group 8 Borylene Complexes

Organometallics, Vol. 28, No. 10, 2009 2951 Table 1 Continued 13 1/2CH2Cl2

18b 1/2C6H5F

index ranges (h, k, l) no. of reflns collected no. of indep reflns/Rint completeness to θmax (%) absorp corr

-16 to 16, -23 to 25, 0 to 34 -23 to 23, -20 to 20, -43 to 43 81 455 57 876 26 654, 0.064 11 086, 0.079 97.0 98.6 semiempirical from equivs

max. and min. transmn refinement method

0.95 and 0.88

no. data/restraints/params goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest peak/hole (e Å-3)

12 969/76/1522 1.136 R1 ) 0.0847, wR2 ) 0.0883 R1 ) 0.1261, wR2 ) 0.1257 1.97 and -1.20

480 Hz). 31P NMR (122 MHz, C6D6): δ 20.1. IR (CD2Cl2 soln, cm-1): ν 1908 (CO). MS(EI): m/z (%) 496.8 (8) (M)+, 468.8 (19) (M - CO)+; exact mass (calcd for M+, 11B isotopomer) m/z 496.7804, (measd) 496.7799. 8 was prepared in a similar manner from 3 and PPh3 and isolated as a yellow-orange solid in comparable yields (ca. 63%). Data for 8: 1H NMR (300 MHz, C6D5CD3): δ 0.84-2.68 (m, 20H, CH2 of Cy), 2.68 (overlapping m, 2H, CH of Cy), 4.34 (s, 5H, Cp), 6.72-7.87 (m, 15H, Ph). 13C NMR (75 MHz, C6D5CD3): δ 26.0, 26.2 (2 overlapping signals), 27.2, 27.7, 27.8, 32.5, 33.4, 33.7, 34.0 (CH2 of NCy2) 59.1, 64.4 (CH of NCy2), 83.3 (Cp), 129.4 (d, 3JCP ) 10 Hz, meta-CH of Ph), 131.9 (paraCH of Ph), 132.8 (d, 2JCP ) 10 Hz, ortho-CH of Ph), 133.8 (d, 1JCP ) 52 Hz, ipso-C of Ph), 221.3 (d, 2JCP ) 35 Hz, CO). 11B NMR (96 MHz, C6D5CD3): δ 61 (br, fwhm ca.720 Hz). 31P NMR (121 MHz, C6D5CD3): δ 82.0. IR (C6D6 soln, cm-1): ν(CO) 1901. MS(EI): m/z (%) 637.1 (weak) M+, 609.1 (10) (M - CO)+, 262 (100) PPh3; exact mass (calcd for M+, 10B isotopomer) m/z 608.2217, (measd) 608.2217. [CpFe(CO)(PMe3){B(NCy2)}]+[BAr f4]- (9), [CpRu(CO)(PMe3){B(NCy2)}]+[BAr f4]- (10), and [CpFe(CO)(PPh3){B(NCy2)}]+[BAr f4]- (11). A solution of 6 (0.17 g, 0.37 mmol) in dichloromethane (2 cm3) was added to a suspension of Na[BAr f4] (1.1 equiv) at 20 °C, and the mixture was sonicated for 20 min, after which the reaction was judged to be complete by 11B NMR and 31P NMR spectroscopy. Filtration, layering with hexanes, and storage at -30 °C led to the formation of 9 as pale yellow crystals (of the dichloromethane solvate) suitable for X-ray diffraction. Isolated yield: 0.13 g, 27%. 1H NMR (300 MHz, CD2Cl2): δ 1.02-2.18 (overlapping m, 20H, CH2 of Cy), 1.61 (d, 2JHP ) 12 Hz, 9H, PMe3), 2.89 (m, 2H, CH of Cy), 4.95 (d, JHP ) 2 Hz, 5H, Cp), 7.57 (s, 4H, para-H of BAr f4-), 7.73 (s, 8H, ortho-H of BAr f4-). 13C NMR (75 MHz, CD2Cl2): δ 22.7(d, 1JCP ) 36 Hz, PMe3), 25.3, 26.3, 26.4, 36.6 (2 overlapping signals) (CH2 of NCy2), 57.5 (CH of NCy2), 84.8 (Cp), 117.8 (para-C of BAr f4-), 124.9 (q, 1JCF ) 276 Hz, CF3 of BAr f4-), 129.2 (q, 1JCF ) 31 Hz, meta-C of BAr f4-), 135.1 (ortho-CH of BAr f4-), 162.1 (q, 1JCB ) 50 Hz, ipso-C of BAr f4-), 270.7 (CO). 11B NMR (96 MHz, CD2Cl2): δ 93 (br, fwhm ca.730 Hz). 19F NMR (283 MHz, CD2Cl2): δ -62.8 (CF3). 31P NMR (121 MHz, CD2Cl2): δ 31.7. IR (CD2Cl2 soln, cm-1): ν(CO) 1981. MS (positive ion electrospray): m/z (%) 416 (100) M+; exact mass (calcd for M+) m/z 416.1975, (measd) 416.1991. Anal. (calcd for 9) C 49.76, H 3.78, N 1.09; (measd) C 49.81, H 3.85, N 1.02. 10 was prepared from 7 in a similar manner and isolated as pale yellow crystals suitable for X-ray diffraction in yields of ca. 30%. Data for 10: 1H NMR (300 MHz, CD2Cl2): δ 1.15-2.10 (overlapping m, 20H, CH2 of Cy), 1.61 (d, 2JHP ) 12 Hz, 9H, PMe3), 2.89 (m, 2H, CH of Cy), 4.95 (d, JHP ) 2 Hz, 5H, Cp), 7.57 (s, 4H, para-H of BAr f4-), 7.73 (s, 8H, ortho-H of BAr f4-). 13C NMR (75 MHz, CD2Cl2): δ 24.0 (d, 1JCP ) 37 Hz, PMe3), 26.3, 26.4, 30.8, 36.7, 36.8 (CH2 of NCy2), 57.5 (CH of NCy2), 88.2 (Cp), 117.8 (para-C of BAr f4-), 124.9 (q, 1JCF ) 276 Hz, CF3 of BAr f4-), 129.2 (q, 2JCF ) 31 Hz, meta-C of BAr f4-), 135.1 (ortho-CH of BAr f4-), 162.1 (q, 1JCB ) 50 Hz, ipso-C of BAr f4-), 199.6 (d, 2JCP ) 17 Hz, CO). 11B NMR (96 MHz, CD2Cl2):

1.16 and 0.90 full matrix least-squares on F2 11 086/0/686 1.030 R1 ) 0.0657, wR2 ) 0.1599 R1 ) 0.1229, wR2 ) 0.1883 1.00 and -1.22

δ 93 (br, fwhm ca.730 Hz). 19F NMR (283 MHz, CD2Cl2): δ -62.8 (CF3). 31P NMR (121 MHz, CD2Cl2): δ 12.7. IR (CD2Cl2 soln, cm-1): ν(CO) 1953. MS(positive ion electrospray): m/z (%) 462 (100) M+; exact mass (calcd for M+, 10B isotopomer) 462.1674, (measd) 462.1706. 11 was prepared from 8 in a similar manner and isolated as a pale yellow microcrystalline solid in yields of ca. 18%. Data for 11: 1H NMR (300 MHz, CD2Cl2): δ 0.87-2.24 (m, 20H, CH2 of Cy), 2.87 (m, 2H, CH of Cy), 5.01 (d, JPH ) 1 Hz, 5H, Cp), 7.32-7.62 (m, 15H, Ph), 7.63 (s, 4H, para-H of BAr f4-), 7.80 (s, 8H, ortho-H of BAr f4-). 13C NMR (75 MHz, CD2Cl2): δ 24.7, 25.9, 26.0, 35.3, 36.0 (CH2 of NCy2), 56.1(CH of NCy2), 86.7 (Cp), 118.8 (para-C of BAr f4-), 126.0 (q, 1JCF ) 275 Hz, CF3 of BAr f4-), 129.4 (d, 2JCP ) 10 Hz, ortho-C of Ph), 130.3 (q, 2JCF ) 34 Hz, meta-C of BAr f4-), 131.9 (para-C of Ph), 132.8 (d, 3JCP ) 10 Hz, meta-C of Ph), 133.8 (d, 1JCP ) 52 Hz, ipso-C of Ph), 136.3 (ortho-CH of BAr f4-), 163.6 (q, 1JCB ) 50 Hz, ipso-C of BAr f4-), 270.3 (CO). 11B NMR (96 MHz, CD2Cl2): δ 92 (br, fwhm ca. 1200 Hz). 19F NMR (283 MHz, CD2Cl2): δ -62.8 (CF3). 31P NMR (121 MHz, CD2Cl2): δ 68.0. IR (CD2Cl2 soln, cm-1): ν(CO) 1988. MS (positive ion electrospray): m/z (%) 602 (100) M+; exact mass: calcd 602.2448, measd 602.2430. meso-[CpFe(CO){B(NCy2)})2(µ-dmpe)]2+[BAr f4]-2 (13). A mixture of 1 (0.15 g, 0.37 mmol) and dmpe (1 equiv) in toluene was photolyzed for 36 h with stirring, after which the reaction was judged to be complete by 11B NMR spectroscopy (complete disappearance of starting material and appearance of a broad signal at δB 63). Volatiles were removed under reduced pressure, and the resulting residue was extracted into pentane (ca. 50 cm3); attempted crystallization invariably led to the formation of an oily residue containing an intractable mixture of products While a single broad 11 B resonance at δB 63 was observed, a number of 31P signals were typically found at δP ∼55. Presumably this reflects, at least in part, the formation of diastereomeric dinuclear boryl complexes of the type rac/mer-[CpFe(CO){B(NCy2)Cl}]2(µ-dmpe) (12; vide infra). Given the greater ease of separation likely for (more crystalline) cationic borylene species, halide abstraction chemistry was therefore carried out on this mixture with the aim of separating the resulting products by fractional crystallization. A solution containing 0.123 g (0.25 mmol, assuming a mixture of [CpFe(CO)B(NCy2)Cl})2(µdmpe)] isomers) in dichloromethane (30 cm3) was added to a suspension of Na[BAr f4] (ca. 1.1 equiv), and the reaction mixture was allowed to stir for 1 h. Volatiles were then removed under reduced pressure, and the residue was extracted into fluorobenzene; layering with hexanes and storage at -30 °C led to the formation of crystals of meso-[CpFe(CO)B(NCy2)})2(µ-dmpe)]2+[BAr f4]-2 (13) as the dichloromethane hemisolvate suitable for X-ray diffraction. Yield: 0.02 g, 19%. Data for 13: 1H NMR (300 MHz, CD2Cl2): δ 0.76-2.18 (overlapping m, 44H, CH2 of Cy and CH2 of dmpe), 1.55, 1.59 (d, 2JHP ) 12 Hz, each 6H, PMe2), 2.96 (m, 4H, CH of Cy), 4.97 (s, 10H, Cp), 7.61 (s, 8H, para-H of BAr f4-), 7.78 (s, 16H, ortho-H of BAr f4-). 13C NMR (75 MHz, CD2Cl2): δ 18.8, 20.2 (br, PMe3), 24.9 (d, 1JCP ) 49 Hz, PCH2), 24.3, 26.3, 31.0, 31.1, 36.7 (CH2 of NCy2), 57.7 (CH of NCy2), 84.9 (Cp), 117.9 (para-C of BAr f4-), 125.0 (q, 1JCF ) 276 Hz, CF3 of BAr f4-),

2952 Organometallics, Vol. 28, No. 10, 2009 Scheme 1.

Synthesis of Cationic Complexes by Halide Abstraction

129.3 (2JCF ) 27 Hz, meta-C of BAr f4-), 135.3 (ortho-CH of BAr f4-), 162.2 (q, 1JCB ) 50 Hz, ipso-C of BAr f4-), 212.4 (CO). 11 B NMR (96 MHz, CD2Cl2): δ 91 (br, fwhm ca. 2400 Hz). 19F NMR (283 MHz, CD2Cl2): δ -62.7 (CF3). 31P NMR (121 MHz, CD2Cl2): δ 45.7 (d, 3JPP ) 15 Hz). IR (CD2Cl2 soln, cm-1): ν(CO) 1979. MS (positive ion electrospray): m/z (%) 415 (100) M2+; exact mass (calcd for M2+) 415.1901, (measd) 415.1891. [CpFe(CO)2C(NCy)2B(NCy)2CNCy2]+[BAr f4]- (17). A mixture of 3 (0.115 g, 0.09 mmol) and dicyclohexylcarbodiimide (2.0 equiv) in dichloromethane (3 cm3) was stirred for 4 h at room temperature, after which time the reaction was judged to be complete by 11B NMR spectroscopy. Layering with hexanes and storage at -30 °C led to the formation of colorless crystals of 17 suitable for X-ray diffraction. Isolated yield: 0.088 g, 65%. 1H NMR (400 MHz, CD2Cl2): δ 1.03-1.93 (m, 60H, CH2 of Cy), 3.07 (m, 2H, CH of NCy2), 3.37 (m, 2H, CH of NCy), 3.44 (m, 2H, CH of NCy), 4.99 (s, 5H, Cp), 7.48 (s, 4H, para-H of BAr f4-), 7.65 (s, 8H, ortho-H of BAr f4-). 13C NMR (75 MHz, CD2Cl2): δ 25.3, 25.4, 25.5 (4CH2 of Cy), 25.5, 25.9, 26.9 (3-CH2 of Cy), 31.4, 33.9, 34.9 (2CH2 of Cy), 55.8 (CH of NCy2), 56.9, 60.0 (CH of NCy), 86.1 (Cp), 117.6 (para-CH of BAr f4-), 124.6 (q, 1JCF ) 274 Hz, CF3 of BAr f4-), 129.0 (2JCF ) 32 Hz, meta-C of BAr f4-), 134.8 (orthoCH of BAr f4-), 161.8 (q, 1JCB ) 52 Hz ipso-C of BAr f4-), 167.1 (guanidinate quaternary), 211.9 (CO), 224.0 (metalla-amidinate quaternary). 11B NMR (96 MHz, CD2Cl2): δ 2.4 (s, fwhm ca. 21 Hz, cation), -7.6 (BAr f4-). 19F NMR (283 MHz, CD2Cl2): δ -62.8 (CF3). IR (CD2Cl2 soln, cm-1): ν(CO) 2040, 1992. MS (positive ion nanoelectrospray): 780.5 (M+, 10%), correct isotope pattern; (negative ion nanoelectrospray): 863.1 (BAr f4-, 100%). Exact mass: (calcd for M+, 54Fe isotopomer) 778.5091, (measd) 778.5101. Anal. (calcd for 17) C 56.22, H 5.09, N 4.26; (measd) C 55.83, H 4.86, N 4.01. [CpFe(CO)2(CNPh)]+[BArf4]- (18b). A mixture of 3 (0.305 g, 0.25 mmol) and PhNCO (5 equiv) in dichloromethane (50 cm3) was stirred for 48 h at room temperature, after which time the reaction was judged to be complete by 11B NMR spectroscopy (complete disappearance of the signal due to 1). Removal of volatiles under reduced pressure, extraction into fluorobenzene, layering with hexanes, and storage at -30 °C led to the isolation of 18b as crystals (of the fluorobenzene hemisolvate) suitable for X-ray diffraction. Yield: 0.116 g, 41%. 1H NMR (300 MHz, CD2Cl2): δ 5.49 (s, 5H, Cp), 7.10-7.52 (overlapping m, 5H, Ph), 7.54 (s, 4H, para-H of BAr f4-), 7.73 (s, 8H, ortho-H of BAr f4-). 13 C NMR (75 MHz, CD2Cl2): δ 87.7 (Cp), 117.9 (para-C of BAr f4-), 125.0 (q, 1JCF ) 273 Hz, CF3 of BAr f4-), 125.6 (orthoCH of Ph), 126.9 (para-CH of Ph), 129.3 (q, 2JCF ) 38 Hz, meta-C of BAr f4-), 130.5 (meta-CH of Ph), 132.2 (ortho-C of BAr f4-), 135.3 (ipso-C of Ph), 162.2 (q, 1JCB ) 55 Hz, ipso-C of BAr f4-), 205.6 (CO). 11B NMR (96 MHz, CD2Cl2): δ -8.8 (BAr f4-). 19F NMR (283 MHz, CD2Cl2): δ -62.7 (CF3). IR (CD2Cl2 soln, cm-1): ν(CN) 2197, ν(CO) 2087, 2052. MS (positive ion electrospray): m/z (%) 280.0 (80%) M+, 252.0 (47%) (M - CO)+, 224.0 (100%) (M - 2CO)+; exact mass (calcd for M+) 280.0055, (measd) 280.0023. Anal. (calcd for 18b · 1/2C6H5F) C 49.37, H 2.07, N 1.18; (measd) C 49.37, H 2.00, N 1.08. In situ ESI-MS monitoring of the reactions of 3 with RCNO (R ) Cy, Ph, 2,6-Xyl) was carried out using the approach reported by Lubben et al.13

Results and Discussion (i) Synthetic Chemistry. Halide abstraction chemistry has proved to be a versatile synthetic route to transition metal

Pierce et al.

complexes containing terminally bound borylene ligands (Scheme 1), allowing the synthesis of a range of complexes with significant variation in the boron-bound substituent (ERn). Thus, for example, the effects of systematic variation in the π-donor properties of ERn are revealed via the synthesis of a series of cationic [(η5-C5R5)Fe(CO)2] complexes featuring the BMes (14), BNCy2 (3), and B(η5-Cp*) (15)5f ligands (Scheme 2) from the corresponding haloboryl complexes, (η5C5R5)Fe(CO)2B(hal)ERn (hal ) Cl, Br; ERn ) Mes, NCy2, Cp*). Moreover, further variation in the nature of the ligand can be brought about by the coordination of a Lewis base to an existing borylene complex, e.g., in the formation of picoline-stabilized system 5. The availability of structure/ bonding and reactivity data for complexes 3, 5, 14, and 15 then provides the basis for a systematic appraisal of the effects of the borylene substituent on electronic structure (vide infra). Although significant enhancements in the stability of cationic metal complexes containing highly unsaturated ligand systems have been achieved by the use of more electron-rich metal centers (as demonstrated by the relative stabilities of [CpFe(CO)2CH2]+ and [CpFe(dppe)CH2]+),9 attempts to systematically vary the metal/ligand framework (LnM) supporting terminal borylene ligands have not been widely reported.3h Synthetic approaches designed to redress this situation, through the synthesis of ruthenium- and/or phosphine-containing systems, are outlined in Schemes 3 and 4. In each case, the borylene substituent (NCy2) has been kept constant to allow for controlled systematic appraisal of the effects of the metal and/or ligand set on electronic structure. Very rare examples of ruthenium terminal borylene complexes have thereby been synthesized from Cy2NBCl2,6l making use of a two-step salt metathesis, halide abstraction protocol analogous to that employed for the corresponding iron systems. Thus, [CpRu(CO)2{B(NCy2)}]+[BAr f4]- (4) can be isolated in 43% overall yield from the dichloroborane and characterized by standard spectroscopic, analytical, and crystallographic methods. Particularly diagnostic are the 11B NMR chemical shift (δB 90) and shift in the measured carbonyl stretching frequencies [2022 and 1957 cm-1; cf. 1990 and 1928 cm-1 for boryl precursor CpRu(CO)2{B(NCy2)Cl}, 2], which is similar to that found for [CpFe(CO)2{B(NCy2)}]+[BAr f4]- (3) [δB 93; ν(CO) 2071, 2028 vs 2000, 1939 cm-1 for CpFe(CO)2{B(NCy2)Cl}, 1]. The 11B NMR shifts for both compounds are within the range expected for terminally bound aminoborylene complexes (δB ca. 85-100).1 These structural inferences were subsequently confirmed crystallographically, with the accompanying structure of aminoboryl precursor 2 offering useful comparison of geometric parameters as a function of metal-ligand bond order (vide infra). Phosphine-iron and -ruthenium systems have also been targeted (Scheme 4), given the known capability of such metal/ ligand frameworks to stabilize related highly electrophilic ligand systems such as terminal dialkylsilylenes. However, in contrast to the markedly less electrophilic charge neutral aminoborylene complexes (OC)5M{BN(SiMe3)2} (M ) group 6 metal) reported by Braunschweig and co-workers,3h simple carbonyl/phosphine exchange cannot be accomplished for cationic complexes 3 and 4. On the contrary, the typical modes of reactivity of cationic aminoborylene systems toward strongly nucleophilic twoelectron donors proceed via coordination at the boron center. Although we have not been able to definitively characterize the products of the reactions of 3 with dppe or PMe3, the analogous


Organometallics, Vol. 28, No. 10, 2009 2953

Scheme 2. Cationic Terminal Borylene Complexes Containing the [(η5-C5R5)Fe(CO)2] Fragment

Scheme 3. Syntheses of Dicarbonyl-Iron and -Ruthenium Borylene Complexesa

a Key reagents and conditions: (i) Cy2NBCl2 (0.9 equiv), diethyl ether, 20 °C, 12 h, 65-68%; (ii) Na[BAr f4] (1.1 equiv), dichloromethane, -78 to 20 °C, 12 h further stirring at 20 °C, 63-80%; (iii) (for 3) 4-picoline (5 equiv), dichloromethane, 20 °C, 1 h, 52%.

Scheme 4. Syntheses of Monosubstituted Phosphine-Iron and -Ruthenium Borylene Complexesa

a Key reagents and conditions: (i) PMe (2 equiv), toluene, UV photolysis (6 h), ca. 33% isolated yield; (ii) Na[BAr f ] (1.1 equiv), dichloromethane, 4 3 -78 to 20 °C, 20 min further sonication at 20 °C, 27%; (iii) and (iv) dmpe (1 equiv), toluene, UV photolysis (36 h), extraction into dichloromethane, Na[BAr f4] (1.1 equiv), -78 to 20 °C, 1 h further stirring at 20 °C, 19%.

reaction with 4-picoline leads to the formation of the B-bound Lewis acid/base adduct [CpFe(CO)2{B(NCy2)(4-pic)}]+[BAr f4](5), which has been characterized by standard spectroscopic, analytical, and crystallographic methods. The 1:1 stoichiometry of adduct formation in 5 was readily established by integration of the Cp CH and picoline CH3 resonances; moreover the upfield shift in the measured 11B NMR resonance (δB 57 vs 93 for 3) and red-shifted carbonyl stretching frequencies (2019 and 1962; cf. 2071 and 2028 cm-1 for 3) are consistent with coordination of the Lewis base to give a three-coordinate boron center. The structure of 5 was subsequently confirmed crystallographically (vide infra). Consequently, in order to isolate phosphine-iron and -ruthenium terminal borylene complexes, introduction of the PR3 ligand at an earlier stage in the synthetic pathway is clearly necessary. Reaction of the boryl precursors

CpM(CO)2{B(NCy2)Cl} (1: M ) Fe; 2: M ) Ru) with the small, strongly σ basic trimethylphosphine (2 equiv) under photolytic conditions leads to the formation of the corresponding mono(phosphine) complexes CpM(CO)(PMe3){B(NCy2)Cl} (6: M ) Fe; 7: M ) Ru) in ca. 35% yield. Compounds 6 and 7 have been characterized by standard spectroscopic techniques, and the structure of 6 was confirmed in the solid state by X-ray crystallography. 31P NMR data for both compounds are in accord with those reported for related complexes [δP 41.5 and 20.1 for 6 and 7, respectively; cf. δP 39.2 and 12.5 for CpFe(CO)(PMe3)Bcat (cat ) 1,2-O2C6H4) and CpRu(CO)(PMe3)Ph, respectively].17,18 Moreover, the slight downfield shifts in the 11B NMR (17) Waltz, K. M.; Muhuro, C. N.; Hartwig, J. F. Organometallics 1999, 18, 3383.


resonances brought about by substitution of a single CO ligand for PMe3 (δB 62 and 57, respectively; cf. 55 and 50 for 1 and 2) mirror that observed for the corresponding Bcat complexes [δB 51.8 and 57.5 for CpFe(CO)2Bcat and CpFe(CO)(PMe3)Bcat, respectively].17 Similar carbonyl substitution chemistry gives access to the related triphenylphosphine complex 8 in ca. 60% yield; the more weakly σ-donating triarylphosphine leads to a noticeably higher carbonyl stretching frequency than that measured for 6 (1901 vs 1890 cm-1). By contrast, the reaction of either 1 or 2 with excess PMe3 does not lead to the formation of useable quantities of CpM(PMe3)2{B(NCy2)Cl}; rather the predominant product isolated in each case results from extrusion of the borylene fragment. Thus, [CpFe(CO)(PMe3)2]+Cl- and CpRu(PMe3)2Cl are the organometallic products isolated from the reactions of 1 and 2, respectively, with excess PMe3 (see Supporting Information).12,19 Although the fate of the [B(NCy2)] fragment has not been definitively established, previous studies have indicated the propensity of borylene ligands ejected from the coordination sphere of group 8 metals to insert into the C-H or C-C bonds of solvent molecules.5b The behavior of boryl complexes 1 and 2 in the presence of excess PMe3 contrasts markedly with that of the related catecholboryl system, CpFe(CO)2Bcat.17 Although in each case the reaction proceeds initially via formation of the mono(phosphine) adduct, only in the case of the Bcat system is a significant yield of the bis(phosphine) boryl complex obtained. These reactivity differences appear to have both electronic and steric origins, reflecting not only the greater steric demands of the B(NCy2)Cl ligand (over Bcat) but also stronger binding of the remaining carbonyl ligand in 6. The IR-measured carbonyl stretching frequencies determined for the mono(phosphine) “intermediates” CpFe(CO)(PMe3)Bcat and 6 (1927 and 1890 cm-1, respectively) are consistent with a greater degree of Fe-C back-bonding in the latter case, and hence with stronger binding of the CO ligand. Presumably this in turn reflects the fact that the B(NCy2)Cl ligand is itself a weaker π acceptor than Bcat, a finding also consistent with the measured carbonyl stretching frequencies for the parent CpFe(CO)2 complexes (2024, 1971 and 2002, 1943 cm-1, for Bcat and B(NCy2)Cl systems, respectively).5e,20 It is also interesting to note that similar chemistry carried out with 1 and the bifunctional phosphine dmpe leads to the formation of the dinuclear boryl complex [CpFe(CO){B(NCy2)Cl}]2(µdmpe), 12, an observation confirmed structurally by X-ray crystallographic studies carried out the corresponding dicationic dinuclear borylene species (13) derived from subsequent halide abstraction (vide infra). Examination of the 3/dmpe reaction mixture by 31P NMR reveals that even after extended reaction times the predominant species present in solution feature a nonchelating dmpe ligand, with only minor resonances being found in the region of the spectrum characteristic of a chelating dmpe ligand (at δP ca. 80 ppm).21 Boryl complexes 6, 7, 8, and 12 prove to be viable substrates for halide abstraction chemistry. Thus, in each case, reaction with Na[BAr f4] in dichloromethane leads to the formation of the corresponding cationic borylene complex typically in 20-30% isolated yield (Scheme 4). The complexes so formed, (18) Lehlkuhl, H.; Schwickardi, R.; Mehler, G.; Kru¨ger, C.; Goddard, R. Z. Anorg. Allg. Chem. 1991, 606, 141. (19) Lehmkuhl, H.; Mauermann, H.; Benn, R. Liebigs Ann. Chem. 1980, 754. (20) Hartwig, J. F.; Huber, S. J. Am. Chem. Soc. 1993, 115, 4908. (21) For a related example, see: Ueno, K.; Hirotsu, M.; Hatori, N. J. Organomet. Chem. 2007, 692, 88.

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[CpM(CO)(PMe3){B(NCy2)}]+[BAr f4]- (9: M ) Fe; 10: M ) Ru), [CpFe(CO)(PPh3){B(NCy2)}]+[BArf4]- (11), and [(CpFe(CO){B(NCy2)})2(µ-dmpe)]2+[BAr f4]-2 (13) have been characterized by standard spectroscopic and analytical techniques, and (in the cases of 9, 10 and 13), the structures in the solid state determined crystallographically. Particularly diagnostic spectroscopic features include the downfield shift in the 11B NMR resonance with respect to the neutral boryl precursor (e.g., δB 93, 93, and 92 for 9, 10, and 11; cf. 55, 57, and 61 for 6, 7, and 8) and carbonyl stretching frequencies (1981, 1953, and 1988 cm-1, respectively), which are blue-shifted with respect to the boryl precursors (1890, 1908, and 1901 cm-1 for 6 and 7), but redshifted compared to the corresponding borylene complexes containing dicarbonyl ancillary ligand sets (2071, 2028 and 2022, 1957 cm-1 for 3 and 4, respectively). While the 1H and 13 C{1H} NMR spectra measured for the boryl precursors 6, 7, and 8 reveal 12 inequivalent cyclohexyl carbons, consistent not only with slow rotation about the BN bond but also with diastereotopic inequivalence within each Cy ring (brought about by the chiral metal center), the corresponding spectra for borylenes 9, 10, and 11 at room temperature reveal only six cyclohexyl resonances. This observation is consistent with rapid rotation about the M-B-N axis on the NMR time scale. In each case a single set of resonances is observed even at -80 °C, although a much greater degree of line broadening for the CH signal is observed in the case of ruthenium borylene complex 10. By applying the line broadening analysis reported by Brookhart, an estimate of the upper limit of the barrier to rotation can be made (∆Grot e 30 kJ mol-1).22 This figure can be compared to a DFT-calculated estimate of 19 kJ mol-1 for CpV(CO)3{BN(SiMe3)2} and reflects not the absolute magnitude of π bonding along the Ru-B-N framework, but rather the difference in such interactions between horizontal and vertical ligand alignments.3d Using a similar line-broadening analysis, an upper limit of 28 kJ mol-1 has been determined by Nelson and co-workers for the closely related vinylidene system [CpRu(Ph2PCHdCH2)2(dCdCH2)]+[PF6]-.24 Finally, the possibility exists for compound 13 to exist in either rac or meso diastereomeric forms due to the presence of two chiral iron centers. The solid state structure of 13 shows that it crystallizes solely as the meso form; moreover NMR spectra of solutions made by dissolving single crystals in CD2Cl2 reveal only a single Cp signal (and one 31P resonance) even after several days at room temperature, implying that epimerization is slow under these conditions. (ii) Structure/Bonding Studies. Synthetic studies have therefore given access to two series of cationic terminal borylene complexes, which offer a means of comparison of electronic structure as a function of either (i) borylene substituent or (ii) (22) The line-broadening analysis used to estimate the rotational barrier for ruthenium borylene complex 9 is adapted from that reported by Brookhart for group 6 carbene complexes: Kegley, S. E.; Brookhart, M.; Husk, G. R. Organometallics 1982, 1, 760 Line broadening of the 13C cyclohexyl CH signal of 10 at-80 °C is ca. 4.1 Hz at half-height (∆W). An estimate of the chemical shift difference (νA-νX) between the resonances associated with the two CH groups in the slow exchange limit is obtained from the two methyl resonances in the related cationic vinylidene complex [CpRe(NO)(PPh3)(dCdCMe2)]+ (∆δC ) 3.7 ppm).23 For 10, νA-νX is thereby estimated as 465 Hz (13C spectra measured at 125.7 MHz), and by using the fast-exchange approximation a minimum value for the rate constant is obtained: k ) (νA-νX) 2/2∆W ) 2.6 × 104 s-1. Thus, from the Eyring equation, ∆Grot e 30 kJ mol-1. The line broadening associated with the corresponding signal for the corresponding iron borylene complex is on the order of 1.8 Hz. (23) Senn, D. R.; Wong, A.; Patton, A. T.; Marsi, M.; Strouse, C. E.; Gladysz, J. A. J. Am. Chem. Soc. 1988, 110, 6096. (24) Hansen, H. D.; Nelson, J. H. Inorg. Chim. Acta 2003, 352, 4.


Figure 1. Molecular structures of (left to right) CpRu(CO)2{B(NCy2)Cl (2) and CpFe(CO)(PMe3){B(NCy2)Cl} · 1/2(C5H12) (6 · 1/2C5H12). Solvate molecule (where appropriate) and hydrogen atoms omitted for clarity; ORTEP ellipsoids set at the 50% probability level. Key bond lengths (Å) and angles (deg): (for 2) Ru(1)-B(11) 2.1412(19), B(11)-N(13) 1.394(2), B(11)-Cl(12) 1.833(2), Ru(1)-B(11)-N(13) 129.76(13), Ru(1)-B(1)-Cl(12) 112.64(10), Cl(12)-B(11)-N(13) 117.32(13); (for 6 · 1/2C5H12) Fe(1)-B(13) 2.023(2), B(13)-N(15) 1.406(3), B(13)-C(14) 1.870(2), Fe(1)-B(13)-N(15) 130.95(15), Fe(1)-B(13)-Cl(14) 114.79(12), Cl(14)-B(13)-N(15) 114.04(15).

metal/ligand fragment. Single-crystal X-ray diffraction studies have been carried out for [CpFe(CO)2{B(NCy2)}]+[BAr f4]- · CH2Cl2 (3 · CH2Cl2),5e [CpRu(CO)2{B(NCy2)}]+[BAr f4]- · (4 · CH 2 Cl 2 ), [CpFe(CO) 2 {B(NCy 2 )(4CH 2 Cl 2 pic)}]+[BAr f4]-(5), [CpFe(CO)(PMe3){B(NCy2)}]+[BAr f4]- · CH2Cl2 (9 · CH2Cl2), [CpRu(CO)(PMe3){B(NCy2)}]+[BAr f4]- (10), and [(CpFe(CO){B(NCy2)})2(µ-dmpe)]2+[BAr f4]-2 · 1/2CH2Cl2 (13 · 1/2CH2Cl2), while that for [Cp*Fe(CO)2{B(Mes)}]+[BAr f4]- · 1/2CH2Cl2 (14 · 1/2CH2Cl2) has been reported previously by us and [CpFe(CO)2{B(η5-Cp*)}]+[AlCl4]- (15) has been reported by Cowley and co-workers.5a,f The X-ray structures of the precursor complexes CpRu(CO)2{B(NCy2)Cl} (2) and CpFe(CO)(PMe3){B(NCy2)Cl} (6) have been obtained for comparative purposes and are reported here for the first time. The structures of these new boryl and borylene complexes are shown in Figures 1-3; crystallographic details are included in Table 1. DFT geometry optimizations and analyses of the σ:π ratio of covalent bonding density for MB and (in relevant cases) BN bonds have been carried out for the cationic components of 3, 4, 5, 9, 10, 14, and 15.15 Salient structural and quantum chemical data for the cations [CpFe(CO)2{B(NCy2)}]+ (3), [CpFe(CO)2{B(NCy2)(4-pic)}]+ (5), [Cp*Fe(CO)2{B(Mes)}]+ (14), and [CpFe(CO)2{B(η5Cp*)}]+ (15), each of which contains the [(η5-C5R5)Fe(CO)2] fragment, are listed in Table 2. In a similar fashion, related data pertaining to the BNCy2 complexes 3, [CpRu(CO)2{B(NCy2)}]+ (4), [CpFe(CO)(PMe3){B(NCy2)}]+ (9), [CpRu(CO)(PMe3){B(NCy2)}]+ (10), and [(CpFe(CO){B(NCy2)})2(µ-dmpe)]2+ (13) are collated in Table 3, which thereby offers a comparison of structural and electronic properties as a function of metal/ ligand fragment. Mesitylborylene complex 14 has previously been reported to feature one of the shortest metal boron bonds yet determined crystallographically.5a A comparison of this bond length [1.792(8) and 1.785(8) Å, for the two crystallographically independent cations] with that measured for the ostensibly single-bonded precursor Cp*Fe(CO)2{B(Mes)Br} [1.972(2) Å] reveals a contraction in the metal-ligand bond length (-9.3%) similar to that found between Fe-C single


bonds and archetypal Fischer carbenes.25 This, taken together with (i) carbonyl stretching frequencies for 14 (2055, 2013 cm-1), which are similar to those reported for [Cp*Fe(CO)2{C(OMe)Me}]+ (2045, 1999 cm-1),26 and (ii) a DFT-calculated σ:π ratio of the covalent bonding density of 62:38 (cf. 64:36 for the model system [CpFe(CO)2(CH2)]+) is consistent with a Fischer carbene-like bonding model for 14, comprising BfFe σ donor and (appreciable) FefB π back-bonding components.5a In the case of cationic aminoborylene complex 3, a similar degree of bond shortening is observed with respect to the boryl precursor [1.859(6) vs 2.053(3) Å, -9.4%]. That said, the significantly longer Fe-B bond compared to that found in 14 (despite the reduced bulk of the cyclopentadienyl ligand) is consistent with the reduced π acidity of the boron center in the presence of the strongly π donor (planar) NCy2 group; the implied reduction in FeB back-bonding is supported by the σ and π symmetry covalent bonding densities (σ:π ) 70:29). The BNR2 ligand present in 3 is, of course, isoelectronic with classical vinylidene systems, and the similar σ:π breakdown (67:33) for the model system [CpFe(CO)2(CCiPr2)]+, together with comparable carbonyl stretching frequencies measured for [CpFe(CO)(PMe3){B(NCy2)}]+ (9, 1981 cm-1) and [CpFe(CO)(PMe3){CC(H)tBu}]+ (1989 cm-1), provides evidence for closely related bonding modes for the two ligand types.27 Consistently, DFT (BLYP/TZP)-calculated Kohn-Sham molecular orbitals for 3 and [CpFe(CO)2(CCiPr2)]+ reflect the presence of similar orthogonal FeB (FeC) and BN (CC) π-bonding systems (Figure 4). Thus, for 3 the HOMO-2 (-9.64 eV) and the LUMO (-5.25 eV) have, respectively, Fe-B π-bonding and antibonding character utilizing the Fe 3dyz and B 2py orbitals. The HOMO and HOMO-1 have B-N π-bonding character, the former having an antibonding phase relationship with Fe 3dxz, the latter formally a bonding alignment; the HOMO-1 therefore features a component that appears to be delocalized along the entire Fe-B-N framework and contributes an additional (albeit minor) Fe-B π-bonding component orthogonal to that represented by the HOMO-2. Further evidence for the strong dependence of the metal ligand bond on the nature of the borylene substituent can be obtained from the crystal structures of the 4-picoline donor-stabilized Cy2NB adduct (5) and of Cowley’s (η5-Cp*)B complex 15.5f Crystallographically determined Fe-B bond lengths [2.049(4) and 1.977(3) Å for 5 and 15, respectively] are significantly longer than those measured for either the MesB or parent Cy2NB complex, reflecting the less π acidic nature of the borylene ligand. Consistent with this, (i) the calculated σ:π ratios (87:13 and 86:14, respectively) and (ii) the measured carbonyl stretching frequencies (2019, 1962 and 2020, 1962 cm-1, respectively, vs 2071, 2028 cm-1 for 3) reflect a much reduced π backbonding component to the metal-boron bond. Presumably a contributory factor to the very long metal-ligand distance measured for 5 is the augmented (trigonal) coordination geometry at boron. Intriguingly, this observation contrasts with that reported very recently for the methylgallylene complexes [(Cp*Ga)4Rh(GaMe)]+[BAr f4]- and [(Cp*Ga)4Rh{GaMe(py)}]+(25) An Fe-C bond length of 1.808(12) has been determined for [CpFe(CO)2(dCCl2)]+: Crespi, A. M.; Shriver, D. F. Organometallics 1985, 4, 1830 This compares with a mean value of 2.09 Å (range 2.036-2.201 Å) determined for [CpFe(CO)2] alkyl complexes from a survey of the Cambridge Structural Database (01/09/2008). (26) Nlate, S.; Guerchais, V.; Lapinte, C. J. Organomet. Chem. 1992, 434, 89. (27) Bullock, R. M. J. Am. Chem. Soc. 1987, 109, 8087.


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Figure 2. Molecular structures of the cationic components of (left to right) [CpFe(CO)2{B(NCy2)}]+[BAr f4]- · CH2Cl2 (3 · CH2Cl2), [CpRu(CO)2{B(NCy2)}]+[BAr f4]- · CH2Cl2 (4 · CH2Cl2), [CpFe(CO)(PMe3){B(NCy2)}]+[BArf4]- · CH2Cl2 (9 · CH2Cl2), [CpRu(CO)(PMe3){B(NCy2)}]+[BArf4]- (10), and [CpFe(CO)2{B(NCy2)(4-pic)}]+[BAr f4]- (5). Counterion, solvate molecule (where appropriate), and hydrogen atoms omitted for clarity; ORTEP ellipsoids set at the 50% probability level. Key bond lengths (Å) and angles (deg): (for 3 · CH2Cl2) Fe(1)-B(2) 1.859(6), B(2)-N(1) 1.324(7), Fe(1)-B(2)-N(1) 178.8(5); (for 4 · CH2Cl2) Ru(1)-B(11) 1.960(6), B(11)-N(12) 1.320(7), Ru(1)-B(11)-N(12) 175.3(5); (for 9 · CH2Cl2) Fe(1)-B(13) 1.821(4), B(13)-N(14) 1.347(5), Fe(1)-P(2) 2.063(4), Fe(1)-B(13)-N(14) 177.7(3); (for 10) Ru(1)-B(13) 1.928(4), B(13)-N(14) 1.345(5), Ru(1)-P(2) 2.306(1), Ru(1)-B(13)-N(14) 170.8(3); (for 5) Fe(1)-B(1) 2.049(2), B(1)-N(1) 1.573(5), B(1)-N(2) 1.391(5), Fe(1)-B(1)-N(1) 112.6(2), Fe(1)-B(1)-N(2) 131.6(3), N(1)-B(1)-N(2) 115.8(3).

Figure 3. Molecular structure of one of the two crystallographically independent centrosymmetric dications in [(CpFe(CO){B(NCy2)})2(µdmpe)]2+[BArf4]-2 · 1/2CH2Cl2 (13 · 1/2CH2Cl2). Second dication, counter-anions, solvate molecule, and hydrogen atoms omitted for clarity; ORTEP ellipsoids set at the 50% probability level. Key bond lengths (Å) and angles (deg): Fe(1)-B(13) 1.830(7), B(13)-N(14) 1.342(8), Fe(1)-P(1) 2.169(2), Fe(1)-B(13)-N(14) 171.2(5).

[BAr f4]-, in which the metal-gallium distance is markedly shorter in the base-stabilized complex [2.334(1) vs 2.471(1) Å].28 This effect (for gallium) has been rationalized in terms of an increased σ donor capability for the gallylene ligand on coordination of the pyridine Lewis base; that bond lengthening is observed in the related borylene systems is consistent with the greater importance of M-B π bonding for the lighter element and with the calculated (significant) reduction in backbonding on coordination of the Lewis base. All-in-all structural, spectroscopic, and computational data are consistent with the strong influence of the borylene substituent on the electronic structure of the metal-ligand bond, which at a superficial level can be likened to the change from a Fischer carbene-like double bond (for BMes complex 14) to a simple donor/acceptor linkage (for 5 and 15). With a view to evaluating further influences on the M-B bond, structural and computational data for a range of cationic BNCy2 complexes containing different metal/ancillary ligand fragments are collected in Table 3. These data, taken with those available for related charge neutral aminoborylene complexes, are consistent with (i) a common model of bonding for the series of cationic and neutral complexes and (ii) a much smaller influence on the electronic structure of the metal-boron bond effected by the metal/ligand fragment (cf. the borylene substituent). Thus, the short metal-boron distances measured for cationic [Cp(28) Cadenbach, T.; Gemel, C.; Zacher, D.; Fischer, R. A. Angew. Chem., Int. Ed. 2008, 47, 3438.

M(CO)2]-containing complexes 3 (M ) Fe) and 4 (M ) Ru) [1.859(6) and 1.960(6), respectively], compared to charge neutral systems of the type LnM{B(N(SiMe3)2)} [LnM ) CpV(CO)3 1.959(6); Cr(CO)5 1.996(6); Mo(CO)5 2.152(2); W(CO)5 2.151(7); Cp*Ir(CO) 1.892(3) Å],3h,i are consistent (i) with changes in the size of the LnM fragment, as assayed by the corresponding M-CO bond lengths in related carbonyl complexes [e.g., d(M-CO) ) 1.816, 1.920, 1.91, 1.915, 2.055, 2.05, and 1.844 Å for [CpFe(CO)3]+, [CpRu(CO)3]+, CpV(CO)4, Cr(CO)6, Mo(CO)6, W(CO)6, and Cp*Ir(CO)2, respectively],29 and thus (ii) with a common mode of interaction for the BNR2 ligand. For 3 and 4 similar percentage reductions in the M-B bond lengths (cf. the respective boryl precursors, -9.4% and -8.5%) and statistically identical BN distances [1.324(7) and 1.320(7) Å, respectively] imply similar degrees of MfB back-bonding for iron and ruthenium systems; consistent with this, only marginal changes in the σ:π ratios of the covalent bonding density are calculated for the two complexes (70:29 and 68:32, respectively) albeit in a direction that is in keeping with the classical π donor capabilities of the 3d and 4d metals.30 Substitution of a single carbonyl ligand by a tertiary phosphine leads to a slight (ca. 2%) shortening of the M-B bond in both iron and ruthenium complexes [e.g., 1.821(4), 1.928(4), and 1.829 (mean) Å for 9, 10, and 13, respectively], although the apparent accompanying BN bond lengthening [1.347(5), 1.345(5), and 1.346 (mean) Å; cf. 1.324(7) and 1.320(7) Å for 3 and 4, respectively] is not statistically significant at the standard 3σ level. Consistent with these minor structural changes, a similar (relatively small) increase in the calculated π component of the MB bond (e.g., 33% for 9 vs 29% for 3) and accompanying decrease for the BN bond (e.g., 33% for 9 vs 36% for 3) is observed on phosphine/carbonyl substitution. (29) (a) Rees, B.; Mitschle, A. J. Am. Chem. Soc. 1976, 98, 7918. [Cr(CO)6]. (b) Mak, T. C. W. Z. Kristallogr. 1984, 166, 277. [Mo(CO)6]. (c) Grevels, F.-W.; Jacke, J.; Klotzbucher, W. E.; Mark, F.; Skibbe, V.; Schaffner, K.; Angermund, K.; Kruger, C.; Lehmann, C. W.; Ozkar, S. Organometallics 1999, 18, 3278. [W(CO)6]. (d) Wilford, J. B.; Whitla, A.; Powell, H. M. J. Organomet. Chem. 1967, 8, 495. [CpV(CO)4]. (e) Gress, M. E.; Jacobson, R. A. Inorg. Chem. 1973, 12, 1746. [CpFe(CO)3+]. (f) Griffith, C. S.; Koutsantonis, G. A.; Raston, C. L.; Selegue, J. P.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 1996, 518, 197. [CpRu(CO)3+]. (g) Chen, J.; Daniels, L. M.; Angelici, R. J. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1993, 49, 1061. [Cp*Ir(CO)2]. (30) For a relevant recent illustration, see: Powell, C. E.; Cifuentes, M. P.; McDonagh, A. M.; Hurst, S. K.; Lucas, N. T.; Delfs, C. D.; Stranger, R.; Humphrey, M. G.; Houbrechts, S.; Asselberghs, I.; Persoons, A.; Hockless, D. C. R. Inorg. Chim. Acta 2003, 352, 9.



Table 2. Crystallographic and Quantum Chemical Parameters Relating to Complexes 3, 5, 14, and 15: Variation in Structure As a Function of Borylene Substituent compound

σ:π breakdown of bonding density/%

ν(CO)/cm-1

∆(FeB)b/%

d(FeB)/Å

[Cp*Fe(CO)2{B(Mes)}]+ (14)

1.792(8)c -9.3 2055, 2013 62:38 1.785(8) [CpFe(CO)2{B(NCy2)}]+ (3) 1.859(6) -9.4 2071, 2028 70:29 d [CpFe(CO)2{B(η5-Cp*)}]+ (15)5f 1.977(3) 2020, 1962 86:14 [CpFe(CO)2{B(NCy2)(4-pic)}]+ (5) 2.049(4) -2.7 to -4.8e 2019, 1962 87:13 a Compounds 3, 5, and 14 isolated as the [BAr f4]- salts, compound 15 as the [AlCl4]- salt. b ∆(FeB) represents the change in Fe-B bond length for the cationic borylene complex compared to the neutral boryl precursor (if available), expressed as a percentage of the bond length of the boryl compound. c For the two crystallographically independent cations. d Data for the boryl precursor not available. e Data for the boryl precursor not available; ∆(FeB) estimated on the basis of the Fe-B distances reported for the tetracoordinate 4-picoline adducts Cp*Fe(CO)2{BCl2(4-pic)} [2.129(3) Å], Cp*Fe(CO)2{BBr2(4-pic)} [2.106(7) Å], and [Cp*Fe(CO)2{BBr(4-pic)2}]+Br- [2.147(2) Å].44 Table 3. Crystallographic and Quantum Chemical Parameters Relating to Complexes 3, 4, 9, 10, and 13: Variation in Structure As a Function of Metal/Ligand Fragment compounda

d(MB)/Å

∆(MB)b/%

d(BN)/Å

∆(BN)b/%

[CpFe(CO)2{B(NCy2)}]+ (3) [CpRu(CO)2{B(NCy2)}]+ (4) [CpFe(CO)(PMe3){B(NCy2)}]+ (9) [CpRu(CO)(PMe3){B(NCy2)}]+ (10) [(CpFe(CO){B(NCy2)})2(µ-dmpe)]2+ (13)

1.859(6) 1.960(6) 1.821(4) 1.928(4) 1.830(7)d 1.828(7)

-9.4 -8.5 -10.0

1.324(7) 1.320(7) 1.347(5) 1.345(5) 1.342(8)d 1.350(8)

-5.2 -5.3 -4.4

c c

c

σ:πbreakdown of bonding density/% 70:29 68:32 67:33 65:35

c

a All compounds isolated as the [BAr f4]- salts. b ∆(MB) represents the change in M-B bond length for the cationic borylene complex compared to the neutral boryl precursor (if available), expressed as a percentage of the bond length of the boryl compound; ∆(BN) represents a similar percentage change in the B-N bond length. c Data not available. d For the two crystallographically independent centrosymmetric dications.

Figure 4. (a) DFT (BLYP/TZP)-calculated (i) HOMO-2 and (ii) LUMO for [CpFe(CO)2{B(NCy2)}]+ showing metal-ligand π and π* character; (iii and iv) analogous orbitals for the model vinylidene complex [CpFe(CO)2(CCiPr2)]+. (b) DFT (BLYP/TZP)-calculated (i) HOMO-1 and (ii) HOMO for [CpFe(CO)2{B(NCy2)}]+ showing BN π character aligned, respectively, in-phase or out-of-phase with the metal 3dxz orbital; (iii and iv) analogous orbitals for the model vinylidene complex [CpFe(CO)2(CCiPr2)]+.

(iii) Oxygen Atom Abstraction. The scope of reactivity of terminal borylene complexes toward archetypal organic substrates has only recently begun to be appreciated. Thus, examples of cycloaddition (involving carbon-carbon and carbon-oxygen multiple bonds),31 metathesis,32 hydride transfer,6e and insertion chemistries33 have recently been reported. Furthermore, given the known (high) thermodynamic stability of the B-O bond and the fact that subvalent boron reagents have previously been shown to be competent for oxygen atom abstraction chemistry,34 (31) For examples of cycloaddition reactivity demonstrated by terminal borylene complexes, see ref 5e, together with: (a) Braunschweig, H.; Herbst, T.; Rais, D.; Seeler, F. Angew. Chem., Int. Ed. 2005, 44, 7461. (b) Braunschweig, H.; Ferna´ndez, I.; Frenking, G.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 5215. (32) For examples of metathesis reactivity demonstrated by terminal borylene complexes, see ref 5c, together with: Braunschweig, H.; Burzler, M.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 8071.

we set out to examine the reactivity of cationic aminoborylene complexes toward CdO double bonds.6e In particular we targeted the oxygen-containing heteroallenes ECO (E ) NR′, O), together with related carbodiimide substrates, R′NCNR′ (Scheme 5). Aminoborylene complex 3 is unreactive toward CO2 in dichloromethane or fluorobenzene, under conditions of either thermal or photolytic activation. Given that previous studies have shown nucleophilic attack at boron to be the key initial step in much of the chemistry of 3,5e we therefore turned our attention (33) For examples of insertion reactivity demonstrated by terminal borylene complexes, see refs 5b and 10 together with: Braunschweig, H.; Dewhurst, R. D.; Herbst, T.; Radacki, K. Angew. Chem., Int. Ed. 2008, 47, 5978. (34) (a) Laitar, D. S.; Mu¨ller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005, 127, 17196. (b) Zao, H.; Lin, Z.; Marder, T. B. J. Am. Chem. Soc. 2006, 128, 15637.


Pierce et al.

Scheme 5. Reactivity of Cationic Aminoborylene Complex 3 toward Heteroallenesa

a Key reagents and conditions: (i) R′NCO (R′ ) Ph, 2,6-Xyl, Cy) (5 equiv), dichloromethane, 20 °C, 48 h, 41% isolated yield of 18b for R′ ) Ph; (ii) CyNCNCy (titration with 11B NMR monitoring), dichloromethane, -30 °C, 24% isolated yield of 16; (iii) (from 3) CyNCNCy (2 equiv), dichloromethane, 20 °C, 4 h, 65%; (iv) for 17: fluorobenzene, 80 °C, 1 week (no reaction).

Figure 5. Molecular structures of the cationic components of [CpFe(CO)2{C(NCy)2B(NCy)2CNCy2}]+[BArf4]- (17) and [CpFe(CO)2(CNPh)]+[BAr f4]- · 1/2C6H5F (18b · 1/2C6H5F). Counterions, solvent molecules (where appropriate), and hydrogen atoms omitted for clarity; ORTEP ellipsoids set at the 50% probability level. Key bond lengths (Å) and angles (deg): (for 17) Fe(1)-C(8) 1.96(2), C(8)-N(1) 1.37(3), C(8)-N(2) 1.35(3), N(1)-B(1) 1.56(3), N(2)-B(1) 1.54(3), B(1)-N(3) 1.55(3), B(1)-N(4) 1.56(3), N(3)-C(21) 1.34(3), N(4)-C(21) 1.37(3), C(21)-N(5) 1.35(3), N(1)-C(8)-N(2) 99(2), N(1)-B(1)-N(2) 83.8(17), N(3)-B(1)-N(4) 84.1(17), N(3)-C(21)-N(4) 104.4(19); (for 18b · C6H5F) Fe(1)-C(58) 1.851(4), C(58)-N(1) 1.158(5), N(1)-C(59)1.400(5),Fe(1)-C(58)-N(1)175.6(3),C(58)-N(1)-C(59) 174.5(4).

to related isocyanate substrates, in the hope that access to the more basic (albeit more sterically hindered) N-centered lone pair would ultimately lead to more facile CdO bond activation. In practice, such an approach proves to be successful, with the reaction of 3 and PhNCO being complete in 48 h at room temperature in dichloromethane. The major organometallic product isolated from this reaction is the phenylisonitrile complex [CpFe(CO)2(CNPh)]+[BAr f4]- (18b), which has been characterized by standard spectroscopic techniques and its identity confirmed by single-crystal X-ray diffraction (Figure 5). In addition, in situ monitoring of the reaction by positive-ion ESI-MS reveals (i) that the formation of [CpFe(CO)2(CNPh)]+ is accompanied by small, but reproducible amounts of [CpFe(CO)3]+ (18a) and (ii) that a cationic species is generated, the molecular ion of which (at m/z ) 606) is characterized by an intensity/time profile appropriate for a

reaction intermediate. Moreover, the measured value of m/z for this peak, together with its isotopic profile and accurate mass, is consistent with a cationic species formed from 3 by assimilating two molecules of PhNCO. Additionally, MS/MS analysis of this cation is consistent with fragmentation pathways that yield the previously identified products [CpFe(CO)2(CNPh)]+ (18b) and [CpFe(CO)3]+ (18a). The corresponding reactions of 3 with 2,6-XylNCO and CyNCO were also monitored by ESIMS and are consistent with the formation of the related isonitile products [CpFe(CO)2(CNR′)]+[BAr f4]- (18c: R′ ) 2,6-Xyl; 18d: R′ ) Cy).35 Despite the fact that intermediate species could be identified by ESI-MS in the reactions of 3 with isocyanate substrates, attempts to isolate such complexes in macroscopic quantities in order to probe their structural and/or reaction chemistry proved unsuccessful. Thus, for example, the reaction of 3 with substoichiometric amounts of PhNCO invariably led to the isolation of the final isonitrile product 18b, together with unreacted 3. As a consequence, the quest for isolated intermediates to model the deoxygenation process turned to reactions with carbodiimides, in the hope that the increased steric bulk afforded by the second N-bound substituent might alter the relative rates of individual mechanistic steps to allow the accumulation of such species in isolable quantities.33 In the event, dicyclohexylcarbodiimide proved to be the most useful substrate in this regard. The reaction of 3 with two or more equivalents of this carbodiimide proceeds to completion over a period of 4 h, leading to the generation of a single boron-containing species, for which the sharp (fwhm ) 21 Hz) 11B NMR resonance at δB 2.4 implies a highly symmetrical four-coordinate boron environment featuring no metal-boron bonds. Spectroscopic and analytical data are consistent with this product being formulated as [CpFe(CO)2C(NCy)2B(NCy)2CNCy2]+[BAr f4]- (17), in which two molecules of the carbodiimide have been assimilated by the parent borylene via insertion into both FedB and BdN double bonds (see Scheme 5). The solid state structure of 17 revealed by single-crystal X-ray diffraction studies (Figure 5) confirms the boronium cation-centered spirocyclic structure (35) The trimeric boron-containing product (Cy2NBO)3 was also identified by 1H and 11B NMR and mass spectroscopy.11c


implied by these insertion steps,36,37 featuring as it does both four-membered guanidinate and metalla-amidinate rings.38 From a mechanistic standpoint, in situ monitoring of the reaction of 3 with 2 equiv of dicyclohexylcarbodiimide by variabletemperature 1H and 11B NMR reveals the presence of two intermediates formed sequentially at -50 and -30 °C, each of which gives rise to a single 11B resonance (at δB 71 and 25, respectively). The first formed (and extremely labile) intermediate is proposed to be the B-bound carbodiimide adduct [CpFe(CO)2{B(NCy2)(CyNCNCy)}]+[BAr f4]- on the basis of the known B-centered Lewis acidity of 35e and an upfield shift in the 11B NMR resonance consistent with previous reports of NfB donor/acceptor adducts formed between 3 (or related cationic borylenes) and compounds containing a CdNR functionality {e.g., δB 64.0 and 53.9 for the structurally characterized imine adducts [CpFe(CO)2{B(X)(iPrNdCMe2)]+[BAr f4]-, X ) OC(H)Ph2 and NiPr2, respectively}.6e Furthermore, the propensity of dicyclohexylcarbodiimide to form donor/acceptor adducts with strongly Lewis acidic boranes prior to further insertion chemistry has previously been demonstrated explicitly by Cowley and co-workers.39 The second-formed intermediate is formulated as [CpFe(CO)2C(NCy)2BNCy2]+[BAr f4]- (16) on the basis of spectroscopic and structural data measured for the isolated compound, which could be obtained as a crystalline solid (free from the double-insertion product 17) by careful addition of a substoichiometric amount of dicyclohexylcarbodiimide. Most tellingly, the 11B chemical shift for 16 (δB 25) is consistent with a product formed by insertion of carbodiimide into the FedB bond, implying, as it does, the absence of any remaining direct Fe-B interaction;40 these inferences were subsequently confirmed crystallographically (see Supporting Information). From intermediate 16 the second insertion step (Scheme 5) appears from NMR monitoring to proceed very rapidly, even at 0 °C, in the presence of excess carbodiimide, to yield 17. Insertion of carbodimides into BdN bonds has previously been reported, with Bu¨rger and co-workers proposing a mechanism involving an initial [2+2] cycloaddition step;41,42 additionally the reaction of CyNCNCy with Cy2NBCl2 is known to cleanly generate Cy2NC(NCy)2BCl2.38a While the ultimate reaction product (17) formed from 3 and dicyclohexylcarbodiimide is thermally robust and a number of related amidinate and guanidinate derivatives of boron have recently been reported,38a,41 no analogous insertion compound could be isolated from the reaction of 3 with isocyanates. In the case of phenylisocyanate, in situ ESI-MS monitoring of the reaction is, however, consistent with the formation of an intermediate with a mass corresponding to 3 plus 2 equiv of (36) For a recent review of boron-centered cations, see: Piers, W. E.; Bourke, S. C.; Conroy, K. D. Angew. Chem., Int. Ed. 2005, 44, 5016. (37) For a related boronium system see: Anderson, K. B.; Franich, R. A.; Kroese, H. F. W.; Meder, R.; Rickard, C. E. F. Polyhedron 1995, 14, 1149. (38) For related guadiniate and metallamidinate systems see: (a) Pierce, G. A.; Coombs, N. D.; Willock, D. J.; Day, J. K.; Stasch, A.; Aldridge, S. Dalton Trans. 2007, 4405. (b) Brunner, H.; Meier, W.; Wachter, J.; Bernal, I.; Raabe, E. J. Organomet. Chem. 1989, 362, 95. (39) Hill, N. J.; Moore, J. A.; Findlater, M.; Cowley, A. H. Chem. Commun. 2005, 5462. (40) For typical 11B chemical shifts of complexes containing M-B single and MdB double bonds, see ref 1d and: Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. ReV. 1998, 98, 2685. (41) (a) Jefferson, R.; Lappert, M. F.; Prokai, B.; Tilley, B. P. J. Chem. Soc. A 1966, 1585. (b) Lu, Z.; Hill, N. J.; Findlater, M.; Cowley, A. H. Inorg. Chim. Acta 2007, 360, 1316. (c) Findlater, M.; Hill, N. J.; Cowley, A. H. Polyhedron 2006, 25, 983. (d) Hill, N. J.; Findlater, M.; Cowley, A. H. Dalton Trans. 2005, 3229. (42) Brauer, D. J.; Bucheim-Spiegel, S.; Bu¨rger, H.; Gielen, R.; Pawelke, G.; Rothe, J. Organometallics 1997, 16, 5321.

Organometallics, Vol. 28, No. 10, 2009 2959 Chart 1

PhNCO. In addition, this intermediate undergoes fragmentation under MS/MS conditions to yield the final products [CpFe(CO)2(CNPh)]+ and [CpFe(CO)3]+. An attractive postulate for the reaction pathway with isocyanates therefore involves formation of a doubly inserted spirocyclic system analogous to 17 (i.e., 17′, Chart 1), followed by scission of the exometalated BNCO ring via either (i) B-N and C-O cleavage (to give the metal isonitrile complex) or (ii) B-O and C-N cleavage (to give the metal carbonyl). In order to probe whether this chemistry does indeed proceed via intermediates similar in structure to 17 (and 16), and with a view to probing the thermodynamic stability of such species, a series of DFT calculations were therefore carried out.16 Using established computational methodology43 we sought to compare the mechanisms for the reactions of the model iron borylene complex [CpFe(CO)2(BNMe2)]+ with the carbodiimide MeNCNMe and the isocyanates R′NCO (R′ ) Me, Ph, 2,6-Xyl). These showed that that both carbodiimide and isocyanate substrates prefer initial insertion into the FedB bond rather than the BdN bond of the borylene complex. Furthermore, mechanistic studies reveal that the carbodiimide reaction ultimately leads to the bis(insertion) compounds [CpFe(CO)2C(NMe)2B(NMe)2CNMe2]+ rather than to the isonitrile systems [CpFe(CO)2(CNMe)]+ (via a net metathesis reaction), on the basis of both thermodynamic (product stability) and kinetic considerations (barrier heights). In the case of isocyanates, the oxygen atom abstraction/ metathesis reaction is shown to be competitive to the insertion pathway. A metathesis reaction is facile for isocyanate substrates (to give [CpFe(CO)2(CNMe)]+ in a single step) if initial coordination at the boron atom occurs via the oxygen donor, which is kinetically favored. On the other hand, insertion chemistry is feasible when the isocyanate attacks initially via the nitrogen donor. However, utilizing the model substrate MeNCO, further reaction of the mono(insertion) product so formed with excess isocyanate was also shown to offer a number of facile (low energetic barrier) routes to [CpFe(CO)2(CNMe)]+, rather than to the formation of the bis(insertion) product [CpFe(CO)2C(NMe)(O)B(NMe)(O)CNMe2]+ (i.e., the direct analogue of the observed spirocyclic product in the carbodiimide reaction, 17′). Although a number of possible intermediate species on the way to [CpFe(CO)2(CNMe)]+ were identified that have a mass corresponding to the initial borylene complex plus two molecules of MeNCO (thereby providing computational corroboration of the results of the ESI-MS monitored reaction with PhNCO), the spirocyclic species 17′ appears not to be a Viable intermediate. Once formed, cleavage of the exometalated BNCO ring via either of the proposed pathways is thermodyanamically unfavorable. Thus, the pathway(s) to the observed isonitrile products appears to compete with that (43) De, S.; Parameswaran, P.; Jemmis, E. D. Inorg. Chem. 2007, 46, 6091. (44) Braunschweig, H.; Radacki, K.; Seeler, F.; Whittell, G. R. Organometallics 2006, 25, 4605.


leading to a bis(insertion) spirocycle, rather than simply being further along the same trajectory. Full details of these mechanistic studies have been included in a companion paper.16

Conclusions Cationic terminal borylene complexes, synthesized by halide abstraction, offer a versatile platform on which to gauge the effects on electronic structure of variation in borylene substituent and metal/ligand framework. While the former leads to stark changes in boron-centered electrophilicity and hence in metal-ligand π character and bond length [e.g., from 1.792(8) to 2.049(4) Å], relatively much smaller changes are effected by changes in the metal/ligand set. Introduction of stronger π donor ruthenium- and/or phosphine-containing fragments is readily brought about by extension of the halide abstraction approach, the latter by substitution of one or both carbonyl ligands at the boryl precursor stage. The effects of substitution at the metal on the M-B and B-N bonds are relatively minor {e.g., Fe-B and B-N bond lengths of 1.821(4), 1.347(5) Å and 1.859(6), 1.324(7) Å for [CpFe(CO)(PMe3){B(NCy2)}]+[BArf4]- and [CpFe(CO)2{B(NCy2)}]+[BArf4]-, respectively}, presumably reflecting, at least in part, the mutually cis disposition of the borylene and phosphine/carbonyl ligands. The utility of cationic complexes containing formally subvalent boron-based ligands in oxygen atom abstraction chemistry has been demonstrated by the conversion of a range of isocyanates to the corresponding (metal-coordinated) isonitriles. Moreover, with a view to modeling potential intermediates, the mechanism of related chemistry between cationic borylenes and carbodiimide substrates has been investigated by structural and

Pierce et al.

in situ ESI-MS approaches. While such studies reveal the formation of a novel spirocyclic boronium complex by a doubleinsertion process, DFT studies imply that the analogous product of isocyanate insertion is unlikely to be an intermediate on the pathway to isonitrile formation. The presence of a number of facile competing reaction pathways and the thermodynamic stability of the spirocyclic product mean that the product distribution is better explained in terms of competing pathways, rather than differing extents of reaction along similar trajectories.

Acknowledgment. We thank the EPSRC for funding this project (GR/F019181/1) and for access to the National Mass Spectrometry and Crystallography Services, Dr. D. J. Willock (Cardiff University) for DFT-based bonding analyses, the Centre for Modeling, Simulation and Design (CMSD), and the High Performance Computing Facility (HPCF) of the University of Hyderabad and the Supercomputer Education and Research Centre (SERC) of the Indian Institute of Science for computational facilities. S.D. gratefully acknowledges research fellowships from CSIR. Supporting Information Available: Complete synthetic, spectroscopic, and crystallographic data for compounds 1, 3, and 16; NMR spectra for 4, 6, and 13; crystallographic data for 17; spectroscopic data for [CpFe(CO)(PMe3)2]+Cl-; CIFs and CheckCIF reports for all new crystal structure determinations. Complete details of the quantum chemical mechanistic studies are included in an associated article.16 This material is available free of charge via the Internet at http://pubs.acs.org. OM801215B

Half-Sandwich Group 8 Borylene Complexes ... - ACS Publications

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