Fe=B double bonds: Synthetic, structural, and reaction chemistry of cationic terminal borylene complexes

Organometallics 2004, 23, 2911-2926

2911

F edB Dou ble Bon d s: Syn th etic, Str u ctu r a l, a n d Rea ction

Ch em istr y of Ca tion ic Ter m in a l Bor ylen e Com p lexes

Deborah L. Coombs, Simon Aldridge,* Andrea Rossin, Cameron J ones, and

David J . Willock

School of Chemistry, Cardiff University, PO Box 912, Park Place, Cardiff, CF10 3TB, U.K.

Received March 22, 2004

Application of halide abstraction chemistry to asymmetric haloboryl complexes (η⁵-C₅-

Me₅)Fe(CO)₂B(ER_n)X leads to the first synthetic route to cationic multiply bonded group 13

diyl species, [(η⁵-C₅Me₅)Fe(CO)₂B(ER_n)]⁺. The roles of steric bulk and π electron release within

the ER_nsubstituent in generating tractable borylene complexes have been probed, as has

the nature of the counterion. A combination of spectroscopic, structural, and computational

techniques leads to the conclusion that the bonding in complexes such as [(η⁵-C₅Me₅)Fe-

(CO)₂B(Mes)]⁺is best described as an FedB double bond composed of BfFe σ donor and

FefB π back-bonding components. An extended study of the fundamental reactivity of

cationic borylene systems reveals that this is dominated not only by nucleophilic addition

at boron but also by iron-centered substitution chemistry leading to overall displacement of

the borylene ligand.

In tr od u ction

geners [e.g., silylenes (R₂Si)²] dominate this area, recent

studies have sought to develop the related chemistry of

The quest for transition metal complexes incorporat-

ing multiply bonded main group ligands has excited

much recent interest, not only due to the fundamental

questions of structure and bonding posed by such

systems but also due to their implication as reagents

or potential catalytic intermediates in important syn-

thetic processes.^1-6Although the chemistries of carbon-

based ligand systems [e.g., alkylidenes or Fischer

carbenes (R₂C)¹] and more recently their heavier con-

group 13 diyl ligands (RE).^3-10Numerous complexes

(5) (a) Bickelhaupt, F. M.; Radius, U.; Ehlers, A. W.; Hoffmann, R.;

Baerends, E. J . New J . Chem. 1998, 26, 1. (b) Ehlers, A. W.; Baerends,

E. J .; Bickelhaupt, F. M.; Radius, U. Chem. Eur. J . 1998, 4, 210. (c)

Radius, U.; Bickelhaupt, F. M.; Ehlers, A. W.; Goldberg, N.; Hoffmann,

R. Inorg. Chem. 1998, 37, 1080. (d) Macdonald, C. L. B.; Cowley, A. H.

J . Am. Chem. Soc. 1999, 121, 12113. (e) Uddin, J .; Boehme, C.;

Frenking, G. Organometallics 2000, 19, 571. (f) Frenking, G.; Fro¨hlich,

N. Chem. Rev. 2000, 100, 717. (g) Boehme, C.; Uddin, J .; Frenking, G.

Coord. Chem. Rev. 2000, 197, 249. (h) Chen, Y.; Frenking, G. Dalton

Trans. 2001, 434. (i) Uddin, J .; Frenking, G. J . Am. Chem. Soc. 2001,

123, 1683. (j) Bollwein, T.; Brothers, P. J .; Hermann, H. L.; Schwert-

feger, P. Organometallics 2002, 21, 5236.

(6) For terminally bound diyl complexes featuring the heavier group

13 elements see, for example: (a) Dohmeier, C.; Loos, D.; Schno¨ckel,

H. Angew. Chem., Int. Ed. 1996, 35, 127. (b) Weiss, J .; Stetzkamp, D.;

Nuber, B.; Fischer, R. A.; Boehme, C.; Frenking, G. Angew. Chem.,

Int. Ed. Engl. 1997, 36, 70. (c) Su, J .; Li, X.-W.; Crittendon, R. C.;

Campana, C. F.; Robinson, G. H. Organometallics 1997, 16, 4511. (d)

Cotton, F. A.; Feng, X. Organometallics 1998, 17, 128. (e) J utzi, P.;

Neumann, B.; Reumann, G.; Stammler, H.-G. Organometallics 1998,

17, 1305. (f) Haubrich, S. T.; Power, P. P. J . Am. Chem. Soc. 1998,

120, 2202. (g) J utzi, P.; Neumann, B.; Reumann, G.; Schebaum, L. O.;

Stammler, H.-G. Organometallics 1999, 18, 2550. (h) Fischer, R. A.;

Weiss, J . Angew. Chem., Int. Ed. 1999, 38, 2830. (i) Uhl, W.; Benter,

M.; Melle, S.; Saak, W.; Frenking, G.; Uddin, J . Organometallics 1999,

18, 3778. (j) Linti, G.; Schno¨ckel, H. Coord. Chem. Rev. 2000, 206-

207, 285. (k) Ueno, K.; Watanabe, T. Tobita, H.; Ogino, H. Organo-

metallics 2003, 22, 4375.

(7) (a) Braunschweig, H.; Wagner, T. Angew. Chem., Int. Ed. Engl.

1995, 34, 825. (b) Braunschweig, H.; Ganter, B. J . Organomet. Chem.

1997, 545, 163. (c) Braunschweig, H.; Mu¨ller, M. Chem. Ber. 1997,

130, 1295. (d) Braunschweig, H.; Kollann, C.; Englert, U. Eur. J , Inorg.

Chem. 1998, 465. (e) Shimoi, M.; Ikubo, S.; Kawano, Y. J . Am. Chem.

Soc. 1998, 120, 4222. (f) Braunschweig, H.; Colling, M. J . Organomet.

Chem. 2000, 614, 18. (g) Braunschweig, H.; Colling, M.; Hu, C.;

Radacki, K. Angew. Chem., Int. Ed. 2002, 41, 1359. (h) Braunschweig,

H.; Colling, M.; Hu, C. H. Inorg. Chem. 2003, 42, 941.

(8) (a) Aldridge, S.; Coombs, D. L.; J ones, C. Chem. Commun. 2002,

856. (b) Coombs, D. L.; Aldridge, S.; J ones, C. Dalton Trans. 2002,

3851. (c) Coombs, D. L.; Aldridge, S.; Coles, S. J .; Hursthouse, M. B.

Organometallics 2003, 22, 4213.

* Corresponding author. E-mail: AldridgeS@cardiff.ac.uk. Tel: +44

(0)2920 875495. Fax: +44 (0)2920 874030.

(1) See, for example: (a) Nugent, W. A.; Mayer, J . M. Metal Ligand

Multiple Bonds; Wiley-Interscience: New York, 1988. (b) Hendon, J .

W. Coord. Chem. Rev. 2003, 243, 3.

(2) See, for example: (a) Grumbine, S. K.; Tilley, T. D.; Arnold, F.

P.; Rheingold, A. L. J . Am. Chem. Soc. 1993, 115, 7884. (b) Grumbine,

S. K.; Tilley, T. D. J . Am. Chem. Soc. 1994, 116, 5495. (c) Weidenbruch,

M.; Stilter, A.; Peters, K.; von Schnering, H. G. Chem. Ber. 1996, 129,

1565. (d) Mitchell, G. P.; Tilley, T. D. J . Am. Chem. Soc. 1997, 119,

11236. (e) Grumbine, S. K.; Mitchell, G. P.; Straus, D. A.; Tilley, T. D.;

Rheingold, A. Organometallics 1998, 17, 5607. (f) Mitchell, G. P.; Tilley,

T. D. J . Am. Chem. Soc. 1998, 120, 7635. (g) Mitchell, G. P.; Tilley, T.

D. Angew. Chem., Int. Ed. 1998, 37, 2524. (h) Tobita, H.; Ishiyama,

K.; Kawano, Y.; Inomata, S.; Ogino, H. Organometallics 1998, 17, 789.

(i) Wanandi, P. W.; Glaser, P. B.; Tilley, T. D. J . Am. Chem. Soc. 2000,

122, 972. (j) Mork, B. V.; Tilley, T. D. J . Am. Chem. Soc. 2001, 123,

9702. (k) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics

2002, 21, 4648. (l) Okazaki, M.; Tobita, H.; Ogino, H. Dalton Trans.

2003, 493. (m) Mork, B. V.; Tilley, T. D. Angew. Chem., Int. Ed. 2003,

42, 357.

(3) (a) Cowley, A. H.; Lomeli, V.; Voight, 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, 2299. (d) Braunschweig, H.; Colling, M.; Kollann, C.; Merz,

K.; Radacki, K. Angew. Chem., Int. Ed. 2001, 40, 4198. (e) Braunsch-

weig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2003,

42, 205. (f) Braunschweig, H.; Colling, M. Eur. J . Inorg. Chem. 2003,

383. (g) Irvine, G. J .; Rickard, C. E. F.; Roper, W. R.; Williamson, A.;

Wright, L. J . Angew. Chem., Int. Ed. 2000, 39, 948. (h) Rickard, C. E.

F.; Roper, W. R.; Williamson, A.; Wright, L. J . Organometallics 2002,

21, 4862.

(9) Several examples of boron-containing clusters featuring facing

capping BR units which may alternatively be described as triply

bridging (µ₃) borylenes are known, for example: Okamura, R.; Tada,

K.; Matsubara, K.; Oshima, M.; Suzuki, H. Organometallics 2001, 20,

4772.

(4) (a) Coombs, D. L.; Aldridge, S.; J ones, C.; Willock, D. J . J . Am.

Chem. Soc. 2003, 125, 6356. (b) Aldridge, S.; Rossin, A.; Coombs, D.

L.; Willock, D. J . Dalton Trans., submitted.

Publication on Web 05/13/2004

2912 Organometallics, Vol. 23, No. 12, 2004

Coombs et al.

have been reported featuring diyl ligands adopting

bridging modes of coordination [e.g., (L_nM)₂(µ-ER)],^7-10

but the synthetic, structural, and reaction chemistry of

analogous compounds containing terminally bound

ligands is less thoroughly understood.^3-6Within this

field, terminal borylene systems (L_nMBR) represent a

very recent and numerically small addition,^3,4with the

highly Lewis acidic boron center typically being stabi-

lized by sterically bulky or π donor R groups or by

coordination of a tethered base. Systematic studies of

the metal-boron bond in two-coordinate borylene com-

plexes such as these have been severely hampered by

the paucity of synthetic routes available and by the

narrow range of compatible metal fragments (predomi-

nantly metal carbonyls). The lack of suitable borane

precursors of the type (RE)_nrules out one of the

principal synthetic approaches available to the heavier

group 13 elements,^6,10and the scope of the salt elimina-

tion methodology used to give access to complexes such

ties between cationic borylene complexes [L_nMBR]⁺and

electrophilic carbenes and silylenes [L_nMER₂]⁺, and

their more strictly isolobal relationship with carbonyl

complexes, [L_nMCO]⁺, we have sought to determine

patterns of reactivity toward both nucleophilic and

multiply bonded reagents.

Exp er im en ta l Section

(i) Gen er a l Con sid er a tion s. All manipulations were car-

ried out under a nitrogen or argon atmosphere using standard

Schlenk line or drybox techniques. Solvents were predried over

sodium wire (hexanes, toluene) or molecular sieves (dichlo-

romethane) and purged with nitrogen prior to distillation from

the appropriate drying agent (hexanes: potassium, toluene:

sodium, dichloromethane: CaH₂). 3,3-Dimethyl-1-butene was

dried over a potassium mirror and Me₂S₂over molecular sieves

prior to use. Benzene-d₆and dichloromethane-d₂(both Goss)

were degassed and dried over potassium (benzene-d₆) or

molecular sieves (dichloromethane-d₂) prior to use. Benzophe-

none, [PPN]Cl ([PPN]⁺) bis(triphenylphosphoranylidene)-

ammonium, [Ph₃PNPPh₃]⁺), [Ph₄P]Br, [ⁿBu₄N]I, Na[BPh₄],

[ⁿBu₄N]BF₄, and Ag[BF₄] (all Aldrich) were dried under high

vacuum (ca. 10^-4Torr) for 12 h prior to use; CO (BOC) was

as (η⁵-C₅Me₅)BFe(CO)₄or (OC)₅WBN(SiMe₃)₂has

been shown to be limited to a very narrow range of boron

dihalide and metal dianion precursors. Although pho-

tolytic borylene transfer has recently been shown to offer

a potentially versatile new synthetic route [e.g., to

previously inaccessible species such as (η⁵-C₅H₅)V(CO)₃-

BN(SiMe₃)₂^3e], structurally characterized terminal bo-

rylene complexes still number no more than a handful.

In recent work we have demonstrated that asym-

metric haloboryl complexes L_nMB(R)X not only are

readily accessible but also prove to be versatile sub-

strates for boron-centered substitution chemistry,^8,11

leading to a range of novel bridging borylene and

asymmetric boryl complexes. Given that halide (or

pseudo-halide) abstraction from coordinated ligand frag-

ments has previously been used to great effect in the

synthesis of low-coordinate or unsaturated group 14

systems,²we speculated that a similar methodology

applied to haloboryl complexes might offer a versatile

new route to terminally bound group 13 diyl complexes.

Such an approach proves to be viable, leading to the

development of a new route to terminal borylene

complexes and to the first examples of cationic group

3a

3b

used without further purification. Na[BAr^f], [PPN][BPh₄], (η⁵-

4

C₅H₅)Fe(CO)₂B(Mes)Br (1), (η⁵-C₅Me₅)Fe(CO)₂B(Mes)Br (2),

(η⁵-C₅H₅)Fe(CO)₂B(C₆H₃Trp₂-2,6)Br (3) (Trp ) 2,4,6-ⁱPr₃C₆H₂),

(η⁵-C₅Me₅)Fe(CO)₂B(Ph)Cl (4), and (η⁵-C₅Me₅)Fe(CO)₂B(NMe₂)-

Br (7) were prepared by literature methods.^8,11-14

NMR spectra were measured on a J EOL 300 Eclipse Plus

FT-NMR spectrometer. Residual protons of solvent were used

for reference for ¹H and ¹³C NMR, while a sealed tube

containing a solution of [ⁿBu₄N][B₃H₈] in CDCl₃was used as

an external reference for ¹¹B NMR. ¹⁹F and ³¹P NMR spectra

were referenced to external CFCl₃and H₃PO₄, respectively.

Infrared spectra were measured for each compound pressed

into a disk with an excess of dried KBr or as a solution in an

appropriate solvent on a Nicolet 500 FT-IR spectrometer. Mass

spectra were measured by the EPSRC National Mass Spec-

trometry Service Centre, University of Wales Swansea. Per-

fluorotributylamine, polyethylenimine, and sodium trifluoro-

acetate were used as the standards for high-resolution EI,

ES+, and ES- mass spectra, respectively. Despite repeated

attempts, satisfactory elemental microanalysis for new boryl

and borylene complexes was frustrated by their extreme air,

moisture, and (in some cases) thermal instability. Character-

ization of the new compounds is therefore based upon multi-

nuclear NMR, IR, and mass spectrometry data (including

accurate mass measurement of molecular ions), supplemented

by single-crystal X-ray diffraction studies in the cases of 5,

10, 12, 14, 15, and 16. In all cases the purity of the bulk

material was established by multinuclear NMR to be >95%.

Abbreviations: st ) strong, md ) medium, b ) broad, s )

singlet, d ) doublet, q ) quartet, pcq ) partially collapsed

quartet, m ) multiplet, fwhm ) frequency width at half-

maximum.

13 diyls [L_nMER]⁺.⁴[(η⁵-C₅Me₅)Fe(CO)₂(BMes)][BAr^f]

4

[Mes ) 2,4,6-Me₃C₆H₂, Ar^f) C₆H₃(CF₃)₂-3,5], for ex-

ample, contains the shortest M-B distance yet reported

(a feature that is indicative of a novel FedB double

bond) and is the first borylene complex containing a

σ-bound hydrocarbyl substituent.^4aHerein we report an

extended investigation of this synthetic approach to both

aryl- and heteroatom-stabilized borylenes, together with

an in-depth study aimed at elucidating fundamental

patterns of reactivity. Although isolated reports of

substitution chemistry for base-stabilized borylenes and

of elegant metal-to-metal borylene transfer chemistry

have appeared in the recent literature,³the fundamen-

tal reactivity of group 13 diyl complexes remains a

largely uncharted area. Given the superficial similari-

(ii) Syn th eses. Syn th esis of [(η⁵-C₅Me₅)F e(CO)₂(BMes)]-

[BAr ^f₄] (5). A solution of (η⁵-C₅Me₅)Fe(CO)₂B(Mes)Br (2) (0.046

g, 0.1 mmol) in CH₂Cl₂(10 cm³) was added slowly to a

suspension of Na[BAr^f] (0.089 g, 0.1 mmol) in CH₂Cl₂(10 cm³)

4

at -78 °C, and the resulting mixture warmed slowly to room

temperature and stirred for 1 h. After this time the reaction

was judged to be complete by ¹¹B NMR spectroscopy, and the

reaction mixture was filtered, layered with hexanes, and cooled

(10) For complexes featuring metal centers linked by diyl ligands

of the heavier group 13 elements see, for example: (a) He, X.; Bartlett,

R. A.; Power, P. P. Organometallics 1994, 13, 548. (b) Cowley, A. H.;

Decken, A.; Olazabal, C. A.; Norman, N. C. Z. Anorg. Allg. Chem. 1995,

621, 1844. (c) Golden, J . T.; Peterson, T. H.; Holland, P. L.; Bergman,

R. G.; Andersen, R. A. J . Am. Chem. Soc. 1998, 120, 223. (d)

Yamaguchi, T.; Ueno, K.; Ogino, H. Organometallics 2001, 20, 501.

(11) Coombs, D. L.; Aldridge, S.; J ones, C. Appl. Organomet. Chem.

2003, 6-7, 356.

(12) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J . J . S.; Smith,

M. D. Inorg. Chem. 2001, 40, 3810.

(13) Pampaloni, G.; Tripepi, G. J . Organomet. Chem. 2000, 593-

594, 19.

(14) Braunschweig, H.; Kollann, C.; Klinkhammer, K. W. Eur. J .

Inorg. Chem. 1999, 1523.

FedB Double Bonds

Organometallics, Vol. 23, No. 12, 2004 2913

to -50 °C for 1 week to yield 5 as colorless crystals suitable

NMR tube at -78 °C. The reaction mixture was warmed to

-40 °C, at which point monitoring by ¹¹B NMR revealed

partial conversion of 7 (δ_B56.0) to 8 (δ_B88.0); further warming

to -20 °C and a 30 min period at this temperature led to

quantitative conversion of 7 to 8, as judged by ¹¹B and ¹H

NMR. Complete NMR characterization of 8 was achieved at

this temperature. ¹H NMR (300 MHz, CD₂Cl₂): δ 1.93 (s, 15H,

1

for X-ray diffraction (0.070 g, 56%). H NMR (300 MHz, CD₂-

Cl₂): δ 2.01 (s, 15H, η⁵-C₅Me₅), 2.34 (s, 3H, para-CH₃), 2.63

(s, 6H, ortho-CH₃), 6.92 (s, 2H, aromatic CH), 7.54 (s, 4H,

para-H of BAr^f^-), 7.70 (s, 8 H, ortho-H of BAr^{f -}). ¹³C NMR

4

(76 MHz, CD₂Cl₂): δ 8.7 (η⁵-C₅Me₅), 20.8 (ortho-CH₃), 21.4

(para-CH₃), 97.1 (η⁵-C₅Me₅quaternary), 116.7 (para-CH of

1

BAr^f^-), 123.8 (q, J _CF) 271 Hz, CF₃of BAr^{f -}), 125.1 (meta-

η⁵-C₅Me₅), 2.83 (s, 6H, NMe₂), 7.55 (s, 4H, para-H of BAr^f^-),

4

CH of mesityl), 128.6 (q, ²J _CF) 31 Hz, meta-C of BAr^{f -}), 134.0

7.70 (s, 8 H, ortho-H of BAr^f^-). ¹³C NMR (76 MHz, CD₂Cl₂):

4

(ortho-CH of BAr^f^-), 147.9, 150.1 (quaternary C’s of mesityl),

δ 10.3 (η⁵-C₅Me₅), 33.3 (NMe₂), 101.8 (η⁵-C₅Me₅quaternary),

4

160.9 (q, ¹J _CB) 49 Hz, ipso-C of BAr^{f -}), 211.4 (CO) (ipso-C of

117.6 (para-CH of BAr^f^-), 124.6 (q, J _CF) 273 Hz, CF₃of

1

4

mesityl ligand not observed). ¹⁹F NMR (283 MHz, CD₂Cl₂): δ

BAr^f^-), 128.8 (q, ²J _CF) 30 Hz, meta-C of BAr^{f -}), 134.8 (ortho-

4

-62.8 (CF₃). ¹¹B NMR (96 MHz, CD₂Cl₂): δ -7.6 (BAr^{f -}),

CH of BAr^f^-), 161.8 (q, J _CB) 50 Hz, ipso-C of BAr^{f -}), 207.6

1

4

145.0 (b, fwhm ca. 750 Hz, BMes). IR (CH₂Cl₂solution, cm^-1):

ν(CO) 2055 st, 2013 st. MS (EI): 349 [(M - CO)⁺, 15%], 321

[(M - 2CO)⁺, 5%], correct isotope distribution for 1 Fe, 1 B

(CO). ¹⁹F NMR (283 MHz, CD₂Cl₂): δ -62.6 (CF₃). ¹¹B NMR

(96 MHz, CD₂Cl₂): δ -7.7 (BAr^f^-), 88.0 (b, fwhm ca. 620 Hz,

4

BNMe₂). Cooling the solution to -80 °C at this point resulted

atoms. MS (ES neg): 863 [BAr^f^-, 100%].

in no observable splitting of the H or ¹³C resonances attribut-

1

4

Rea ction of (η⁵-C₅Me₅)F e(CO)₂B(Mes)Br (2) w ith Na -

[BP h ₄]. A solution of 2 (0.030 g, 0.07 mmol) in CH₂Cl₂(4 cm³)

was added slowly to a suspension of Na[BPh₄] (0.022 g, 0.07

mmol) in CH₂Cl₂(2 cm³) at -78 °C, and the resulting mixture

was warmed slowly to room temperature and stirred for 16 h.

After this time, the reaction was judged to be complete by the

disappearance of the resonance due to 2 and the appearance

of a single peak at δ_B66.0. The compound giving rise to this

resonance was subsequently shown to be BPh₃by comparison

of spectroscopic data with those reported previously.¹⁵Volatiles

were removed in vacuo, and the remaining solid was extracted

with hexanes (5 cm³). Cooling this solution to -50 °C yielded

orange crystals of (η⁵-C₅Me₅)Fe(CO)₂Ph, the identity of which

was confirmed by comparison of spectroscopic data with those

reported previously.¹⁶

able to the NMe₂group (singlets at δ_H2.74 and δ_C32.5,

respectively). The reaction mixture was then warmed back to

-20 °C and filtered while cold into a solution of [PPN]Cl (0.150

g, 0.25 mmol) in CD₂Cl₂(1 cm³). After warming to room

temperature and stirring for 5 min the reaction mixture was

analyzed spectroscopically, revealing clean and complete

conversion of 8 to (η⁵-C₅Me₅)Fe(CO)₂B(NMe₂)Cl (9). 9 was

identified by comparison of spectroscopic data with those

reported previously by Braunschweig:^{7d 1}H NMR (300 MHz,

CD₂Cl₂) δ 1.73 (s, 15H, η⁵-C₅Me₅), 2.92 (s, 3H, NMe₂), 3.03 (s,

3H, NMe₂). ¹³C NMR (76 MHz, CD₂Cl₂): δ 9.7 (η⁵-C₅Me₅), 41.2

(NMe₂), 43.8 (NMe₂), 95.5 (η⁵-C₅Me₅quaternary), 217.5 (CO).

¹¹B NMR (96 MHz, CD₂Cl₂): δ 58.6. IR (CD₂Cl₂solution, cm^-1):

ν(CO) 1983 st, 1923 st.

Syn th esis of (η⁵-C₅Me₅)F e(CO)₂B(Mes)Cl (10) fr om 5

a n d [P P N]Cl. Details of the synthesis of 10 from 5 and [PPN]-

Cl have previously been reported by us.^8cData are reproduced

here solely for comparative purposes. In addition, after our

earlier report we have managed to obtain X-ray quality

crystals of 10 by repeated recrystallization from hexanes at

-30 °C. Details of this structure are therefore reported here

for the first time. Characterizing data for 10: ¹H NMR (300

MHz, C₆D₆): δ 1.35 (s, 15H, C₅Me₅), 2.20 (s, 3H, para-CH₃),

2.28 (s, 6H, ortho-CH₃), 6.74 (s, 2H, aromatic CH). ¹³C NMR

(76 MHz, C₆D₆): δ 9.0 (η⁵-C₅Me₅), 21.0 (para-CH₃), 21.6 (ortho-

CH₃), 97.1 (η⁵-C₅Me₅quaternary), 127.7 (aromatic CH), 133.2,

136.2 (aromatic quaternary), 216.2 (CO). ¹¹B NMR (96 MHz,

C₆D₆): δ 112.1 (b, fwhm ca. 580 Hz). IR (KBr disk, cm^-1):

ν(CO) 1996 md, 1936 md. MS (EI): M⁺) 412 (5%), isotopic

pattern corresponding to 1 Fe, 1 B, 1 Cl atoms, strong fragment

ion peaks at m/z 384 [(M - CO)⁺, 60%] and 356 [(M - 2CO)⁺,

100%]. Exact mass: calcd 412.1058, measd 412.1055. In

addition, spectroscopic data for the hexane-insoluble product

of this reaction are consistent with those reported previously

Attem p ted Bor ylen e Syn th eses Usin g Oth er Ar yl-

(h a lo)b or yl P r ecu r sor s a n d H a lid e Ab st r a ct ion R e-

a gen ts. The reactions of (η⁵-C₅H₅)Fe(CO)₂B(Mes)Br (1), (η⁵-

C₅H₅)Fe(CO)₂B(C₆H₃Trp₂-2,6)Br (3), and (η⁵-C₅Me₅)Fe(CO)₂B-

(Ph)Cl (4) with Na[BAr^f] and of 2 with Ag[BF₄] were carried

4

out using a common method. Thus for example, a solution of

3 (0.048 g, 0.06 mmol) in CH₂Cl₂(5 cm³) was added slowly to

a suspension of Na[BAr^f] (0.056 g, 0.06 mmol) in CH₂Cl₂(5

4

cm³) at -78 °C. The resulting mixture was warmed slowly to

room temperature and stirred for 1 h. After this time the ¹¹B

NMR resonance due to 3 (δ_B114.9) had disappeared, but no

new resonances attributable to borylene or boryl products were

observed. In situ monitoring of the halide abstraction process

by multinuclear NMR revealed that complete consumption of

the boryl starting material is typically achieved very rapidly

even at -70 °C; even at such temperatures it proved impossible

to detect any products containing Fe-B bonds.

Rea ction of (η⁵-C₅Me₅)F e(CO)₂B(NMe₂)Br (7) w ith Na -

[BAr ^f₄]. A solution of 7 (0.054 g, 0.14 mmol) in CH₂Cl₂(4 cm³)

for [PPN][BAr^f].¹⁷

was added slowly to a suspension of Na[BAr^f] (0.123 g, 0.14

4

mmol) in CH₂Cl₂(2 cm³) at -78 °C. The resulting mixture was

warmed slowly to room temperature and stirred for 1 h. After

this time the ¹¹B NMR resonance corresponding to 7 (δ_B) 56.0)

had disappeared and had been replaced by a single broad peak

at δ_B88.0 (fwhm ca. 620 Hz). Monitoring of the progress of

the reaction by infrared spectroscopy revealed the accompany-

ing growth of two new carbonyl bands at 2050 (st) and 2006

(st) cm^-1. The reaction mixture was filtered, layered with

hexanes, and cooled to -50 °C, but the compounds isolated

from repeated crystallizations were shown by ¹¹B NMR to be

hydrolysis products.

Syn th esis of 2 fr om 5 a n d [P h ₄P ]Br . To a solution of

[Ph₄P]Br (0.045 g, 0.1 mmol) in CD₂Cl₂(2 cm³) was added a

solution of 5 (0.040 g, 0.03 mmol) in CD₂Cl₂(2 cm³). The

resulting mixture was stirred at room temperature for 2 h,

after which the ¹¹B resonance due to 5 (δ_B145.0) was found to

be completely converted into a single signal at δ_B113.0

characteristic of 2. Further comparison of multinuclear NMR

and IR data (for the isolated compound) with those obtained

for an authentic sample of 2 in CD₂Cl₂confirmed the identity

of 2 as the sole boron-containing product.^8b

Rea ction of 5 w ith [ⁿBu ₄N]I. To a solution of [ⁿBu₄N]I

(0.049 g, 0.13 mmol) in CH₂Cl₂(3 cm³) was added a solution

of 5 (0.161 g, 0.13 mmol) in CH₂Cl₂(6 cm³). The reaction

mixture was stirred for 2 h, at which point ¹¹B NMR monitor-

ing revealed complete conversion of 5 (δ_B145.0) to a single

boron-containing species at δ_B110.7. Filtration, removal of

Gen er a tion of [(η⁵-C₅Me₅)F e(CO)₂B(NMe₂)][BAr ^f₄] (8)

a n d Tr a p p in g w ith [P P N]Cl. A solution of 7 (0.020 g, 0.05

mmol) in CD₂Cl₂(2 cm³) was added slowly to a suspension of

Na[BAr^f] (0.046 g, 0.05 mmol) in CD₂Cl₂(0.5 cm³) in a Young’s

4

(15) Odom, J . D.; Moore, T. F.; Goetze, R.; No¨th, H.; Wrackmeyer,

B. J . Organomet. Chem. 1979, 173, 15.

(16) J acobsen, S. E.; Wojcicki, A. J . Am. Chem. Soc. 1973, 95, 6962.

(17) Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.; Adamson, D. R.;

Bergman, R. G. J . Am. Chem. Soc. 2002, 124, 1400.

2914 Organometallics, Vol. 23, No. 12, 2004

Coombs et al.

volatiles in vacuo, and recrystallization from hexanes (ca. 10

cm³) at -30 °C initially yielded a small quantity (,0.010 g) of

a dark red crystalline solid subsequently identified crystallo-

graphically as (η⁵-C₅Me₅)Fe(CO)₂I (13) (full crystallographic

details are included in the Supporting Information).¹⁸Subse-

quent cooling of the supernatant solution to -50 °C then

yielded pale yellow crystals of (η⁵-C₅Me₅)Fe(CO)₂B(Mes)I (11)

(0.038 g, 56%). Characterizing data for 11: ¹H NMR (300 MHz,

C₆D₆): δ 1.26 (s, 15H, η⁵-C₅Me₅), 2.22 (s, 6H, ortho-CH₃), 2.24

(s, 3H, para-CH₃), 6.70 (s, 2H, aromatic CH). ¹³C NMR (76

MHz, C₆D₆): δ 9.9 (η⁵-C₅Me₅), 20.9 (para-CH₃), 21.1 (ortho-

CH₃), 95.5 (η⁵-C₅Me₅quaternary), 130.6 (aromatic CH), 129.0,

136.4 (aromatic quaternary), 216.3 (CO). ¹¹B NMR (96 MHz,

C₆D₆): δ 110.7. IR (KBr disk, cm^-1): ν(CO) 2005 st, 1955 st.

MS (EI): fragment ion peaks at m/z 476 [(M - CO)⁺, 25%],

448 [(M - 2CO)⁺, 5%]. Periodic sampling of the reaction

mixture by infrared spectroscopy revealed, in addition to bands

attributable to 11 [ν(CO) 2005 and 1955 cm^-1], weaker

shoulders at 2017 and 1969 cm^-1, which were present even at

short reaction times (ca. 30 min) and whose relative intensities

(with respect to the bands at 2005 and 1955 cm^-1) appeared

to vary little. For comparative purposes (η⁵-C₅Me₅)Fe(CO)₂I

Rea ction of 5 w ith CO. A solution of 5 (0.217 g, 0.18 mmol)

in CH₂Cl₂(20 cm³) was stirred for 6 h under a continuous flow

of CO, after which the reaction was judged to be complete by

¹¹B NMR (by the complete replacement of the peak due to the

starting material with a single peak at δ_B17). Volatiles were

removed in vacuo and the remaining solid washed successively

with hexanes, toluene, and dichloromethane (ca. 10 cm³of

each). No boron-containing products could be detected in any

of the washings by ¹¹B NMR, thus implying significant

volatility for the species giving rise to the resonance at δ_B17

in the original reaction mixture. The red dichloromethane-

soluble fraction was filtered, layered with hexanes, and cooled

to -30 °C, whereupon red crystals of [(η⁵-C₅Me₅)Fe(CO)₃]-

[BAr^f] (14) suitable for X-ray diffraction were isolated (0.053

4

1

g, 75%). H NMR (300 MHz, C₆D₆): δ 2.00 (s, 15H, η⁵-C₅Me₅),

7.56 (s, 4H, para-H of BAr^f^-), 7.71 (s, 8H, ortho-H of BAr^{f -}).

4

¹³C NMR (76 MHz, C₆D₆): δ 9.8 (η⁵-C₅Me₅), 104.5 (η⁵-C₅Me₅

1

quaternary), 117.5 (para-CH of BAr^f^-), 124.6 (q, J _CF) 273

4

Hz, CF₃of BAr^{f -}), 128.9 (q, J _CF) 31 Hz, meta-C of BAr^f),

2

4

134.8 (ortho-CH of BAr^f^-), 161.8 (q, J _CB) 53 Hz, ipso-C of

1

4

BAr^f^-), 204.0 (CO). ¹⁹F NMR (283 MHz, CD₂Cl₂): δ -62.7

4

(CF₃). ¹¹B NMR (96 MHz, CD₂Cl₂): δ -7.6 (BAr^{f -}). IR (CH₂Cl₂

4

(13) in CH₂Cl₂has been reported to give rise to bands at 2019

solution, cm^-1): ν(CO) 2105 st, 2046 st. MS (ES pos): M⁺

)

18

and 1971 cm^-1

;

in addition those measured for a sample of

275 (100%), isotopic pattern corresponding to 1 Fe atom. MS

13 prepared independently from [(η⁵-C₅Me₅)Fe(CO)₂]Na and

(ES neg): 863 [BAr^f^-, 100%]. Exact mass (cation): calcd

4

I₂are at 2017 and 1971 cm^-1

.

275.0365, measd 275.0365.

Syn th esis of (η⁵-C₅Me₅)F e(CO)₂B(Mes)F (12) fr om 5

a n d [ⁿBu ₄N]BF ₄. To a solution of [ⁿBu₄N]BF₄(0.056 g, 0.17

mmol) in CH₂Cl₂(3 cm³) was added slowly a solution of 5 (0.212

g, 0.17 mmol) in CH₂Cl₂(6 cm³). The reaction mixture was

stirred for 2 h, at which point ¹¹B NMR monitoring revealed

complete conversion of 5 to a single boron-containing species

at δ_B90.4. Filtration, removal of volatiles in vacuo, and

recrystallization from hexanes (ca. 10 cm³) at -50 °C yielded

yellow crystals of 12 suitable for X-ray diffraction (0.043 g,

63%). ¹H NMR (300 MHz, C₆D₆): δ 1.49 (s, 15H, η⁵-C₅Me₅),

2.16 (s, 3H, para-CH₃), 2.43 (s, 6H, ortho-CH₃), 6.73 (s, 2H,

aromatic CH). ¹³C NMR (76 MHz, C₆D₆): δ 9.4 (η⁵-C₅Me₅), 21.0

(para-CH₃), 21.6 (ortho-CH₃), 96.3 (η⁵-C₅Me₅quaternary), 128.8

Rea ction of 5 w ith 3,3-Dim eth yl-1-bu ten e. To a solution

of 3,3-dimethyl-1-butene (0.015 g, 0.18 mmol) in CH₂Cl₂(2 cm³)

was added slowly a solution of 5 (0.223 g, 0.18 mmol) in CH₂Cl₂

(8 cm³). The reaction mixture was stirred for 16 h, at which

point complete consumption of the starting material 5 was

confirmed by ¹¹B NMR. Volatiles were removed in vacuo, and

the remaining solid was washed successively with hexanes,

toluene, and dichloromethane (ca. 10 cm³of each). No boron-

containing products could be detected in any of the washings

by ¹¹B NMR. The orange dichloromethane-soluble fraction was

filtered, layered with hexanes, and cooled to -30 °C, where-

upon deep yellow crystals of [(η⁵-C₅Me₅)Fe(CO)₂(η²-H₂CdCH-

^tBu)][BAr^f] (15) suitable for X-ray diffraction were isolated

4

3

1

t

(aromatic CH), 135.9 (d, J _CF) 3.5 Hz), 136.9 (aromatic

(0.125 g, 63%). H NMR (300 MHz, C₆D₆): δ 1.06 (s, 9H, Bu),

quaternary), 216.7 (CO). ¹⁹F NMR (283 MHz, CD₂Cl₂): δ 56.2

1.82 (s, 15H, η⁵-C₅Me₅), 2.68 (d, J _HH) 8.3 Hz (cis), 1H,

3

1

(pcq). ¹¹B NMR (96 MHz, C₆D₆): δ 90.4 (b, d, J _BF) 180 Hz).

^tBuHCdCH₂), 3.11 (d, J _HH) 15.1 Hz (trans), 1H, BuHCd

3

t

The resonance observed in the ¹¹B{¹H} spectrum of 12 in C₆D₆

room temperature was broad, with coupling to the ¹⁹F nucleus

not completely resolved. Warming to 60 °C however allows the

two components of the doublet to be resolved and the coupling

constant to be determined (¹¹B{¹H} spectra at 20, 40, and 60

°C are included in the Supporting Information). IR (KBr disk,

cm^-1): ν(CO) 1989 st, 1931 st. MS (EI): M⁺) 396 (5%), isotopic

pattern corresponding to 1 Fe, 1 B atoms, fragment ion peaks

at m/z 368 [(M - CO)⁺, 15%], 340 [(M - 2CO)⁺, 100%], and

325 [(M - 2CO - Me)⁺, 30%]. Exact mass: calcd 396.1354,

measd 396.1356.

3

t

CH₂), 3.99 (dd, J _HH) 8.3 (cis), 15.1 Hz (trans), 1H, BuHCd

CH₂), 7.55 (s, 4H, para-H of BAr^f^-), 7.71 (s, 8H, ortho-H of

4

BAr^f^-). ¹³C NMR (76 MHz, C₆D₆): δ 9.8 (η⁵-C₅Me₅), 30.2

4

(quaternary C of ^tBu), 35.6 (CH₃of ^tBu), 98.7 (η⁵-C₅Me₅

quaternary), 102.5 (CH₂of alkene), 117.5 (para-CH of BAr^f^-),

4

124.6 (q, J _CF) 272 Hz, CF₃of BAr^{f -}), 128.9 (q, J _CF) 32

1

2

4

Hz, meta-C of BAr^f^-), 134.7 (CH of alkene), 134.9 (ortho-CH

4

of BAr^f^-), 161.7 (q, ¹J _CB) 50 Hz, ipso-C of BAr^{f -}), 213.1 (CO).

4

¹⁹F NMR (283 MHz, CD₂Cl₂): δ -62.7 (CF₃). ¹¹B NMR (96

MHz, CD₂Cl₂): δ -7.6 (BAr^f^-). IR (CH₂Cl₂solution, cm^-1):

4

ν(CO) 2033 md, 1997 md. MS (ES pos): M⁺) 331 (50%),

isotopic pattern corresponding to 1 Fe atom, fragment ion

peaks at m/z 275 [(M - 2CO)⁺, 100%], 247 [(M - alkene)⁺,

Rea ction of 5 w ith [P P N][BP h ₄]. To a solution of [PPN]-

[BPh₄] (0.150 g, 0.18 mmol) in CH₂Cl₂(8 cm³) was added slowly

a solution of 5 (0.217 g, 0.18 mmol) in CH₂Cl₂(2 cm³) at -78

°C and the resulting mixture warmed slowly to room temper-

ature and stirred for 1 h. After this time, the reaction was

judged to be complete by the disappearance of the resonance

due to 5 and the appearance of a single peak at δ_B66.0. The

compound giving rise to this resonance was subsequently

shown to be BPh₃by comparison of spectroscopic data with

those reported previously.¹⁵Volatiles were removed in vacuo,

and the remaining solid was extracted with hexanes (5 cm³).

Cooling this solution to -50 °C yielded orange crystals of (η⁵-

C₅Me₅)Fe(CO)₂Ph, the identity of which was confirmed by

comparison of spectroscopic data with those reported previ-

ously.¹⁶

40%]. MS (ES neg): 863 [BAr^f^-, 100%]. Exact mass (cation):

4

calcd 331.1355, measd 331.1352.

Rea ction of 5 w ith Ben zop h en on e. To a solution of

benzophenone (0.028 g, 0.15 mmol) in CH₂Cl₂(2 cm³) was

added slowly a solution of 5 (0.191 g, 0.15 mmol) in CH₂Cl₂(8

cm³). The reaction mixture was stirred for 3 h, after which

the reaction was judged to be complete by ¹¹B NMR (by the

replacement of the peak due to the starting material with a

single peak at δ_B17). Volatiles were removed in vacuo, and

the remaining solid was washed successively with hexanes,

toluene, and dichloromethane (ca. 10 cm³of each). No boron-

containing products could be detected in any of the washings

by ¹¹B NMR. The red dichloromethane-soluble fraction was

filtered, layered with hexanes, and cooled to -30 °C, where-

upon red crystals of [(η⁵-C₅Me₅)Fe(CO)₂(η¹-OdCPh₂)][BAr^f]

4

(18) Akita, M.; Terada, M.; Tanaka, M.; Morooka, Y. J . Organomet.

Chem. 1996, 510, 255.

(16) suitable for X-ray diffraction were isolated (0.109 g, 56%).

FedB Double Bonds

Organometallics, Vol. 23, No. 12, 2004 2915

¹H NMR (300 MHz, C₆D₆): δ 1.75 (s, 15H, η⁵-C₅Me₅), 7.39 (m,

of dichloromethane, were located and refined in the asym-

metric unit. There were no significant geometric differences

found between the two independent cations or the two inde-

pendent anions. The relatively high R-factor for the crystal

structure arises from the fact that the crystal chosen for the

experiment was found to diffract weakly at high angle. This

is despite the crystal being relatively large (0.35 × 0.35 × 0.15

mm) and of relatively good quality. Prior to the collection of

data on this crystal, data collections on a number of other

samples of the compound were attempted, but all samples

showed similar problems. Despite the poor quality of the data

all indicators point toward a well-behaved structure and the

molecular connectivity is unambiguous. A number of the

fluorine atoms within the CF₃groups on both anions showed

high anisotropic temperature factors, suggesting disorder

within these groups. Attempts were made to model this

disorder in all such CF₃groups with limited success. The

disorder in three of the CF₃groups was successfully modeled,

but in all others it was not possible to do so, presumably due

to dynamic disorder within these groups. The fluorine atoms

on these groups were, however, refined anisotropically.

2H, para-H of benzophenone), 7.56 (m, 8H, overlap of meta-H

of benzophenone and para-H of BAr^f^-), 7.73 (m, 12H, overlap

4

of ortho-H of benzophenone and ortho-H of BAr^f^-). ¹³C NMR

4

(76 MHz, C₆D₆): δ 9.3 (η⁵-C₅Me₅), 98.0 (η⁵-C₅Me₅quaternary),

1

117.5 (para-CH of BAr^f^-), 124.6 (q, J _CF) 272 Hz, CF₃of

4

BAr^f^-), 128.3 (para-C of benzophenone), 128.7 (q, J _CF) 27

2

4

Hz, meta-C of BAr^f^-), 129.1 (meta-C of benzophenone), 129.4

4

(ortho-C of benzophenone), 134.0 (ortho-CH of BAr^f^-), 136.4

4

1

(ipso-C of benzophenone), 161.8 (q, J _CB) 50 Hz, ipso-C of

BAr^f^-), 210.1 (CO of benzophenone), 215.1 (metal bound CO).

4

¹⁹F NMR (283 MHz, CD₂Cl₂): δ -62.7 (CF₃). ¹¹B NMR (96

MHz, CD₂Cl₂): δ -7.6 (BAr^f^-). IR (KBr, cm^-1): ν(CO) 2050

4

md, 2004 md, 1612 md (benzophenone). MS (ES pos): M⁺

)

429 (75%), isotopic pattern corresponding to 1 Fe atom,

fragment ion peaks at m/z 401 [(M - CO)⁺, 20%], 373 [(M -

2CO)⁺, 30%]. MS (ES neg): 863 [BAr^f^-, 100%]. Exact mass

4

(cation): calcd 429.1153, measd 429.1149.

Th er m olysis of 5 in Dich lor om eth a n e Solu tion . A

solution containing 5 (0.152 g, 0.12 mmol) in CD₂Cl₂(1 cm³)

was transferred to a flame-dried Young’s NMR tube. ¹¹B{¹H}

1

and H spectra were measured at room temperature and then

(iv) Com p u ta tion a l Meth od ology. The computational

approach utilized for geometry optimization and bond dis-

sociation energy (BDE) calculations on 5, 8 and [(η⁵-C₅H₅)Fe-

(PMe₃)₂(BMes)]⁺was identical to that reported previously for

similar studies on metal complexes containing boron-based

ligand systems.^4b,23The ADF2002.03 code was employed,

periodically over a period of ca. 24 h at 40 °C. The relative

concentration of 5 in solution was assessed by integration of

the signal at δ_H2.63 due to the six ortho methyl protons of

the mesityl substituent. The procedure was repeated in

CH₂Cl₂, using a sealed internal deuterium lock. First-order

rate constants were then obtained from the (linear) plots of

ln([5]/[5]_o) versus time.

together with BLYP exchange-correlation functionals;^24,25

a

basis set derived from Slater-type orbitals with a triple-ú

valence shell and two polarization functions per atom was used

in all cases, except for [(η⁵-C₅H₅)Fe(PMe₃)₂(BMes)]⁺, where,

due to the larger number of atoms, a smaller basis set was

chosen for the peripheral hydrogen and carbon atoms of the

PMe₃ligands (double-ú valence shell with one polarization

function per atom). The frozen core approximation was em-

ployed for atoms other than hydrogen. For the atoms of the

second period (B, C, N, O) the core orbitals were defined as

1s, while for phosphorus and iron orbitals up to the 2p and

3p, respectively, were included in the core. All structures were

fully optimized based on unrestricted electronic structure

calculations with complexes in the singlet ground state.

Ch em ica l Tr a p p in g of t h e Mesit ylb or ylen e F r a g-

m en t: Rea ction of 5 w ith Me₂S₂. To a solid sample of 5

(0.206 g, 0.17 mmol) was added a solution of Me₂S₂(0.262 g,

0.28 mmol) in hexanes and the reaction mixture stirred at

room temperature for 2 h. At this point the ¹¹B NMR spectrum

of the hexane-soluble fraction showed a single strong resonance

at δ_B66.0, characteristic of MesB(SMe)₂(17).¹⁹Filtration and

removal of volatiles in vacuo led to the isolation of a colorless

oil, which was confirmed to be MesB(SMe)₂by further com-

parison of spectroscopic data with those reported previously.¹⁹

(iii) Gen er a l Cr ysta llogr a p h ic Meth od . Data for com-

pounds 5, 10, 12, 13, 14, 15, and 16 were collected on an Enraf

Nonius Kappa CCD diffractometer; data collection and cell

refinement were carried out using DENZO and COLLECT,^20,21

and structure solution and refinement using SHELXS-97 and

SHELXL-97, respectively.²²With the exception of (η⁵-C₅Me₅)-

Fe(CO)₂I (13) 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 (figures for

compounds 14 and 15 are included in the Supporting Informa-

tion), and complete details of each structure have been

deposited with the CCDC (numbers as listed in Table 1). In

addition complete details for each structure (including CIF

files) have been included in the Supporting Information. The

structure of compound 13 has previously been reported,¹⁸and

the crystallographic study undertaken here merely served to

unambiguously confirm the identity of a minor (yet chemically

significant) side product isolated in low yield. Details of this

structure are included in the Supporting Information.

The bond dissociation energy for given MB bonds was

calculated by estimating the energy change associated with

the bond cleavage reaction:

[(η⁵-C₅R₅)Fe(L)₂BX]⁺f [(η⁵-C₅R₅)Fe(L)₂]⁺+ BX

The structures of the products from this reaction were

independently optimized to allow for the inclusion of any

molecular relaxation.²³The σ and π contributions to the

bonding energies were determined in the same way as reported

previously for transition metal boryl complexes.²³In each case

a single-point calculation was carried out with the optimized

structure aligned so that the MdB bond was along the z-axis.

A specially developed analysis program was then used to carry

out an adapted Mulliken analysis, which separates the bonding

density into contributions from atomic orbitals of σ and π

The crystal structure of compound 5 was solved and refined

in the monoclinic space group C2/c. Two crystallographically

independent cations and anions, in addition to one molecule

(23) Dickinson, A. A.; Willock, D. J .; Calder, R. J .; Aldridge, S.

Organometallics 2002, 21, 1146.

(19) Siebert, W.; Schaper, K.-J .; Schmidt, M. J . Organomet. Chem.

1970, 25, 315.

(20) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter,

C. W., Sweet, R. M., Eds.; Academic Press: New York, 1996; Vol. 276,

p 307.

(21) COLLECT, Data collection software; Nonius B.V., 1999.

(22) Sheldrick, G. M. SHELX97: Programs for Crystal Structure

Analysis (Release 97-2); University of Go¨ttingen: Go¨ttingen, Germany,

1998.

(24) (a) ADF2002.03 was obtained from SCM, Theoretical Chemis-

try, Vrije Universiteit: Amsterdam, Netherlands, http://www.scm.com.

(b) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J . A.; Fonseca

Guerra, C.; Baerends, E. J .; Snijders, J . G.; Ziegler, T. J . Comput.

Chem. 2001, 22, 931. (c) Fonseca Guerra, C.; Snijders, J . G.; te Velde,

G.; Baerends, E. J . Theor. Chem. Acc. 1998, 99, 391.

(25) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Lee, C.; Yang,

W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.

2916 Organometallics, Vol. 23, No. 12, 2004

Coombs et al.

Ta ble 1. Cr ysta llogr a p h ic Da ta for Com p ou n d s 5, 10, 12, 14, 15, a n d 16

5^.1/2CH₂Cl₂

10

_53.5H₃₉B₂ClF₂₄FeO_{2 21}H₂₆BClFeO₂

12

C₂₁H₂₆BFFeO₂

empirical formula

CCDC deposit number

fw

temperature (K)

wavelength (Å)

cryst syst

C

213762

1282.77

150(2)

210381

412.53

120(2)

229790

396.08

150(2)

0.71073

monoclinic

C2/c

36.301(7)

25.675(5)

23.956(5)

90

93.81(3)

0.71073

monoclinic

P2₁/c

9.5840(2)

17.3711(3)

12.9997(2)

90

111.249(1)

90

2017.11(6)

4

0.71073

triclinic

P1h

space group

unit cell dimens: a (Å)

b (Å)

8.0000(16)

8.6770(17)

14.650(3)

92.67(3)

101.17(3)

97.57(3)

986.3(3)

2

c (Å)

R (deg)

â (deg)

γ (deg)

90

22278(8)

16

volume (Å³)

Z

density(calc) (Mg/m³)

1.530

1.358

1.334

abs coeff (mm^-1

)

0.440

0.892

0.785

F(000)

10320

864

416

cryst color, habit

colorless plate

0.35 × 0.35 × 0.15

2.91 to 25.02

-43 to 43, -30 to 27, -28 to 28

34 393

19 345 (0.0553)

98.3

Sortav

0.9370, 0.8613

full matrix least squares on F²

19 345/111/1560

1.030

R1 ) 0.0888, wR2 ) 0.1974

R1 ) 0.1679, wR2 ) 0.2356

0.756 and -0.561

yellow plate

cryst size (mm³)

0.40 × 0.20 × 0.08

0.20 × 0.15 × 0.10

θ range for data colln (deg)

index ranges (h, k, l)

no. of reflns collected

no. of indep reflns/R_int

completeness to θ_max(%)

absorp corr

max. and min. transmn

refinement method

no. of data/restraints/params

goodness-of-fit on F²

final R indices [I>2σ(I)]

R indices (all data)

3.27 to 27.48

3.43 to 26.34

-12 to 12, -22 to 22, -16 to 16

35 998

4627 (0.0516)

-9 to 9, -10 to 10, -18 to 18

12 594

3933 (0.0907)

98.1

Sortav

0.9856 to 0.4788

full matrix least squares on F²

3933/0/244

99.8

semiempirical from equivalents

0.8321, 0.7168

full matrix least squares on F²

4627/0/244

1.030

1.041

R1 ) 0.0286, wR2 ) 0.0713

R1 ) 0.0353, wR2 ) 0.0754

0.319 and -0.313

R1 ) 0.0446, wR2 ) 0.1059

R1 ) 0.0544, wR2 ) 0.1111

0.811 and -0.567

largest diff peak and hole

(e Å^-3

)

14

15

16^.1/4C₆H₁₄

empirical formula

CCDC deposit number

fw

C

₄₅H₂₇BF₂₄FeO₃

C

₅₀H₃₉BF₂₄FeO₂

C_58.5H_40.5BF₂₄FeO₃

229793

1314.07

229791

1138.33

229792

1194.47

temperature (K)

wavelength (Å)

cryst syst

150(2)

0.71073

monoclinic

150(2)

0.71073

orthorhombic

150(2)

0.71073

monoclinic

space group

unit cell dimens: a (Å)

b (Å)

P2₁/c

12.286(3)

19.347(4)

Pbca

17.935(4)

19.412(4)

P2₁/n

21.5502(3)

12.5448(2)

c (Å)

20.360(4)

29.048(6)

22.8344(5)

R (deg)

90

â (deg)

104.19(3)

90

4691.9(16)

4

1.612

0.457

2272

106.4300(10)

90

5921.04(18)

4

1.474

0.373

2650

γ (deg)

volume (Å³)

Z

10 113(4)

8

1.569

0.427

density(calc) (Mg/m³)

absorp coeff (mm^-1

F(000)

)

4816

cryst color, habit,

yellow plate

yellow cube

red cube

cryst size (mm³)

0.20 × 0.20 × 0.10

0.35 × 0.30 × 0.25

0.25 × 0.20 × 0.18

θ range for data colln (deg)

index ranges (h, k, l)

no. of reflns collected

no. of indep reflns/R_int

completeness to θ_max(%)

absorp corr

2.95 to 25.35

2.97 to 25.34

3.07 to 26.04

-14 to 14; -23 to 23; -24 to 24

31 959

8579 (0.0413)

99.7

-21 to 21, -23 to 23, -34 to 34

50 616

9212 (0.0866)

99.6

-26 to 26; -15 to 15; -28 to 28

47 003

11 596 (0.0682)

99.1

Sortav

max. and min. transmn

refinement method

no. of data/restraints/params

goodness-of-fit on F²

final R indices [I>2σ(I)]

R indices (all data)

0.9557, 0.9141

full matrix least squares on F²

8579/36/712

0.933, 0.692

0.953 and 0.876

full matrix least squares on F²

11 596/168/974

1.039

R1 ) 0.0547, wR2 ) 0.1392

R1 ) 0.0867, wR2 ) 0.1520

0.492 and -0.316

full matrix least squares on F²

9212/624/712

1.035

1.025

R1 ) 0.0814, wR2 ) 0.2196

R1 ) 0.1007, wR2 ) 0.2358

0.931 and -0.606

R1 ) 0.0744, wR2 ) 0.1938

R1 ) 0.1151, wR2 ) 0.2180

1.326 and -0.495

largest diff peak and hole

(e Å^-3

)

symmetry.^23,26In addition total Mulliken charges for all atoms

and Mayer bond orders were obtained for comparison with the

ADF output to ensure the accuracy of the analysis program.

Resu lts a n d Discu ssion

(i) Syn th etic Ch em istr y. Asymmetric haloboryl

complexes 1-4 have proved to be versatile substrates

for boron-centered substitution chemistry, leading to a

(26) Dickinson, A. A. Ph.D. Thesis, Cardiff University, 2003.

FedB Double Bonds

Organometallics, Vol. 23, No. 12, 2004 2917

Sch em e 1. Su bstitu tion a n d Abstr a ction Ch em istr y of Asym m etr ic Ha lobor yl Com p lexes, L_nM-B(ER_n)X

Sch em e 2. Syn th eses of Asym m etr ic

Sch em e 3. Syn th esis of Ca tion ic Mesitylbor ylen e

Com p lex [(η⁵-C₅Me₅)F e(CO)₂(BMes)][BAr ^f₄] (5) by

Br om id e Abstr a ction ^a

Ar yl(h a lo)bor yl Com p lexes 1-4^a

a

Reaction conditions: (i) Na[BAr^f] 1 equiv, dichlo-

4

romethane, -78 to 18 °C, 1 h.

Me₅)Fe(CO)₂unit.^{27 1}H and ¹³C NMR data for 5 confirm

the presence of mesityl, (η⁵-C₅Me₅), and CO fragments,

and mass spectra display peaks consistent with [M -

a

Reaction conditions: (i) Na[(η⁵-C₅R₅)Fe(CO)₂] 1 equiv,

toluene, 18 °C, 4-96 h.

CO]⁺and [M - 2CO]⁺(EI) and [BAr^f]^-ions (ES-).

range of novel bridging borylene and asymmetric boryl

complexes.^8,11Given that abstraction of anionic frag-

ments from coordinated ligands has previously been

used to great effect in the synthesis of low-coordinate

or unsaturated systems (e.g., silylenes and germylenes²),

we speculated that a similar methodology applied to

haloboryl complexes might offer a versatile new route

to terminally bound group 13 diyl complexes (Scheme

1). Although relatively electron-rich phosphine-contain-

ing metal fragments have typically been employed, for

example in the synthesis of terminal silylenes {e.g., [(η⁵-

C₅Me₅)Ru(PMe₃)₂(dSiMe₂)]^{+ 2b}}, our initial attempts to

produce analogous cationic terminal borylene complexes

have utilized the more readily accessible [(η⁵-C₅R₅)Fe-

(CO)₂]⁺moiety (R ) H, Me).

4

Additionally, the strongly downfield ¹¹B NMR shift (δ_B

145.0) is similar to that observed previously for iron

complexes containing mesitylborylene ligands, albeit

adopting a µ₂bridging mode of coordination {cf. δ_B158.0

and 161.9 for [(η⁵-C₅H₅)Fe(CO)₂]₂(µ-BMes) and trans-

[(η⁵-C₅H₅)Fe(CO)]₂(µ-BMes)(µ-CO), respectively}.^{8b 11}

B

shifts in the range δ_B87-204 have previously been

reported by Braunschweig for terminal borylene com-

plexes containing metals from groups 5, 6, and 8;^3b-f

lower field values have typically been obtained for

borylene ligands containing weakly π donor substitu-

ents. The measured ¹¹B shift for 5 is therefore consistent

with a terminally bound BMes ligand, a structural

inference that has subsequently been confirmed by

X-ray crystallography (vide infra).

Asymmetric haloboryl complexes incorporating aryl

functions of varying steric bulk (e.g., 1-4) have been

shown in previous work to be readily and cleanly

accessible by simple substitution chemistry (Scheme

2).^8,11Thus, such species appeared to represent suitable

precursors for the halide abstraction chemistry outlined

in Scheme 1. In practice, both the steric properties of

the putative borylene substituent (and of the metal

fragment) and the inherent coordinating properties/

reactivity of the counterion employed appear to play key

roles in the isolation (or otherwise) of stable, tractable

cationic borylene complexes. Thus, attempted halide

abstraction from complexes 1, 3, and 4, which contain

steric shielding at either the boron or the iron center

(but not both), leads to the generation of no observable

borylene products even at -78 °C. In the case of boryl

precursor 2, however, which contains sterically bulky

substituents at both metal and boron centers [i.e., both

Attempts to generate similar cationic complexes using

the commercially available halide abstraction reagents

Na[BPh₄] and Ag[BF₄] proceed with rapid and complete

consumption of the starting material 2, but lead to the

isolation of no tractable borylene products. In the case

of Ag[BF₄] spectroscopic data are inconsistent with the

presence of either boryl or borylene species in solution.

Additionally, subsequent studies have revealed that the

highly Lewis acidic boron center in [(η⁵-C₅Me₅)Fe(CO)₂-

(BMes)]⁺is incompatible with the tetrafluoroborate

counterion; reaction of 5 with [ⁿBu₄N][BF₄] generates

the neutral fluoroboryl complex (η⁵-C₅Me₅)Fe(CO)₂B-

(Mes)F in high yield by fluoride exchange (vide infra).

By contrast, the reaction of 2 with a suspension of

Na[BPh₄] in dichloromethane leads to the generation

of BPh₃and (η⁵-C₅Me₅)Fe(CO)₂Ph (6) (Scheme 4).

A possible pathway for the formation of these products

involves initial removal of bromide from boryl precursor

2, yielding the cationic borylene as the tetraphenylbo-

rate salt, i.e., [(η⁵-C₅Me₅)Fe(CO)₂(BMes)][BPh₄]; ab-

straction of phenyl anion from tetraphenylborate and

displacement of the mesitylborylene ligand would then

yield the observed products. Displacement of the mesi-

tylborylene fragment from the coordination sphere of

the [(η⁵-C₅Me₅)Fe(CO)₂]⁺unit by nucleophiles is found

to be one of its principal modes of reactivity (vide infra),

mesityl and (η⁵-C₅Me₅) groups], reaction with Na[BAr^f],

4

filtration, layering with hexanes, and cooling to -50 °C

for one week led to the isolation of 5 as colorless crystals

in yields of ca. 55% (100 mg scale) (Scheme 3).^4a

IR-measured carbonyl stretching frequencies for 5

(2055 and 2013 cm^-1) are shifted to significantly higher

wavenumber compared to the starting boryl complex 2

(2006 and 1961 cm^{-1 8b}

presence of a cationic complex containing the (η⁵-C₅-

)

and are consistent with the

2918 Organometallics, Vol. 23, No. 12, 2004

Coombs et al.

Sch em e 4. Rea ction of Br om obor yl Com p lex 2

w ith Na [BP h ₄]^a

a

Reaction conditions: (i) Na[BPh₄] 1 equiv, dichlo-

romethane, -78 to 18 °C, 30 min.

and consistent with the proposed mechanism, indepen-

dent reaction of the [BAr^f]^-salt of [(η⁵-C₅Me₅)Fe(CO)₂-

4

(BMes)]⁺(5) with [PPN][BPh₄] (a dichloromethane-

soluble [BPh₄]^-source) leads to very rapid generation

of BPh₃and (η⁵-C₅Me₅)Fe(CO)₂Ph. Further details of

this and other reactions of complex 5 are outlined below.

In an attempt to extend the halide abstraction meth-

odology to the synthesis of further cationic borylene

complexes, we have examined the reactivity of amino-

(bromo)boryl complex (η⁵-C₅Me₅)Fe(CO)₂B(NMe₂)Br (7)

with Na[BAr^f]. 7 is readily synthesized from Me₂NBBr₂

4

according to literature procedures¹⁴and reacts readily

with Na[BAr^f] in dichloromethane at -20 °C. The

4

complex so generated is characterized by two ¹¹B NMR

resonances, a sharp signal at δ_B-7.7 characteristic of

the [BAr^f]^-anion and a broad signal at δ_B88.0. The

4

position of the latter resonance is very similar to those

reported previously for charge neutral terminal ami-

noborylene complexes [e.g., δ_B98.3, 92.3, 86.6, and 88.2

for L_nMBN(SiMe₃)₂, L_nM ) (η⁵-C₅H₅)V(CO)₃, (OC)₅Cr,

(OC)₅W, and (OC)₄Fe, respectively^3b,c,e,f]. Additionally

IR-measured carbonyl stretching frequencies for the

new species (2050 and 2006 cm^-1) are similar to those

reported for 5, being similarly indicative of a cationic

[(η⁵-C₅Me₅)Fe(CO)₂]⁺fragment.²⁷

F igu r e 1. ¹¹B{¹H} NMR monitoring of the conversion of

(η⁵-C₅Me₅)Fe(CO)₂B(NMe₂)Br (7) to [(η⁵-C₅Me₅)Fe(CO)₂-

(BNMe₂)][BAr^f] (8) and ultimately (η⁵-C₅Me₅)Fe(CO)₂B-

4

(NMe₂)Cl (9) in CD₂Cl₂. The spectra reproduced are (a) 7

at 18 °C prior to mixing with Na[BAr^f]; (b) a mixture of 7

4

and 8 at -40 °C, 30 min after mixing of 7 with Na[BAr^f];

4

(c) 8, after a further 30 min reaction time at -20 °C; and

(d) 9, after subsequent addition of [PPN]Cl to the reaction

mixture and warming to room temperature.

In our hands, attempts to isolate this complex as a

pure solid material have met with no success, due to

its extreme moisture and air sensitivity. NMR monitor-

(see Figure 1), with complete conversion of 7 to 8 being

ing of the reaction of 7 with Na[BAr^f] in CD₂Cl₂over

achieved by reaction with 1 equiv of Na[BAr^f] at -20

4

the temperature range -70 to -20 °C, however, allows

the new complex (8) to be synthesized in solution in very

high purity; complete conversion of 7 to 8 is achieved

by warming the reaction mixture to -20 °C (see Figure

1), and full multinuclear NMR data for 8 can thus be

obtained. These data are consistent with the presence

°C and chloroboryl complex 9 subsequently being formed

quantitatively on addition of [PPN]Cl and warming to

room temperature. Identification of final product 9 was

made by comparison of its spectroscopic data with those

reported previously by Braunschweig.^7d

The observation of equivalent methyl groups within

the amino substituent of 8 even at -80 °C (sharp

singlets at δ_H2.74 and δ_C32.5 at -80 °C) contrasts with

the situation found in boryl complexes 7 and 9,^7d,14but

is consistent with that found for the analogous SiMe₃

groups in (η⁵-C₅H₅)V(CO)₃BN(SiMe₃)₂.^3eAs such, these

data are consistent either with a static structure in

which the methyl groups are related by a plane of

symmetry encompassing the (η⁵-C₅Me₅) centroid, Fe, B,

and N atoms, or with a freely rotating NMe₂unit and a

low barrier to rotation about the Fe-B bond. The

feasibility of both of these possibilities has been dem-

onstrated by DFT calculations. A very low barrier to

rotation (ca. 2.2 kcal mol^-1) has been predicted around

the V-B bond in the model vanadium aminoborylene

complex (η⁵-C₅H₅)V(CO)₃BNH₂;^3eadditionally the mini-

mum energy conformation of the cation [(η⁵-C₅Me₅)Fe-

(CO)₂(BNMe₂)]⁺has been calculated to feature a (η⁵-

of the [BAr^f]^-counterion, with (η⁵-C₅Me₅) and CO

4

ligands at the metal center, and with equivalent nitrogen-

bound methyl substituents. In addition, further char-

acterization of the new complex was attempted by

examining the products of its reaction with [PPN]Cl in

CD₂Cl₂. The reaction of cationic mesitylborylene com-

plex 5 with the same reagent has been shown to

generate (η⁵-C₅Me₅)Fe(CO)₂B(Mes)Cl by chloride addi-

tion to the boron center (vide infra). Hence the formation

of the known compound (η⁵-C₅Me₅)Fe(CO)₂B(NMe₂)Cl

(9)^7dwould provide further evidence that the new

species (8) is the [BAr^f]^-salt of the cationic aminobo-

4

rylene [(η⁵-C₅Me₅)Fe(CO)₂(BNMe₂)]⁺(Scheme 5). In

practice this reaction proceeds very cleanly as expected

(27) See, for example: Nlate, S.; Guerchais, V.; Lapinte, C. J .

Organomet. Chem. 1992, 434, 89. (b) McArdle, P.; MacHale, D.;

Cunningham, D.; Manning, A. R. J . Organomet. Chem. 1991, 419, C18.

FedB Double Bonds

Organometallics, Vol. 23, No. 12, 2004 2919

Sch em e 5. Gen er a tion a n d Tr a p p in g of Ca tion ic Am in obor ylen e Com p lex

[(η⁵-C₅Me₅)F e(CO)₂(BNMe₂)][BAr ^f₄] (8)^a

a

Reaction conditions: (i) Na[BAr^f] 1 equiv, d₂-dichloromethane, -20 °C, 30 min; (ii) [PPN]Cl ca. 5 equiv, d₂-dichloromethane,

4

-20 to 18 °C, 30 min.

PPh₃)]⁺[2.381(2) Å].^2b,eThe shortening of the Fe-B

distance in 5 compared to Fe-B σ-type bonds is there-

fore as expected for a ligand containing a first-row

element and is consistent with the presence of an Fed

B double bond.

Tilley has proposed that the RudSi double bond in

the cationic silylene complex [(η⁵-C₅Me₅)Ru(PMe₃)₂(d

SiMe₂)]⁺is composed of a SifRu donor/acceptor σ

component supplemented by RufSi π back-bonding into

the vacant Si-based p orbital.^2bA similar bonding

arrangement for 5 would imply a significantly greater

FefB back-bonding component than is found in (η⁵-C₅-

Me₅)BFe(CO)₄.^3a,5dSuch an arrangement is conceivable

given the highly electrophilic nature of the boron center

in 5 and the significantly higher DFT-predicted π

bonding capability of the BPh ligand compared to B(η⁵-

C₅H₅).⁵ⁱThe presence of an appreciable FefB back-

bonding component for 5 is implied by carbonyl stretch-

ing frequencies (2055, 2013 cm^-1) in excess of those of

[(η⁵-C₅Me₅)Fe(CO)₂{dCMe(OMe)}]⁺(2045, 1999 cm^-1).^27a

That these values are still short of those reported for

[(η⁵-C₅Me₅)Fe(CO)₃]⁺(2105, 2045 cm^-1), however, al-

most certainly reflects the stronger σ donor nature of

BR over CO.^27bStronger σ donor properties than CO

have been predicted by Hoffmann and Baerends for the

borylene ligands BX (X ) F, NH₂, NMe₂) on the basis

of DFT calculations. Such properties are thought to

reflect the higher HOMO energy for the boron-based

ligands.^5a-c

The orientation of the mesityl fragment in 5 [torsion,

(η⁵-C₅Me₅) centroid-Fe(1)-C(1)-C(2) ) 91.3(6)°] is

such that an FefB π interaction involving the HOMO

of the [(η⁵-C₅Me₅)Fe(CO)₂]⁺fragment³²could populate

one of the two perpendicular vacant p orbitals at boron,

with the other being stabilized by π interaction with the

mesityl ring. Consistent with this, the distance B(1)-

C(1) [1.491(10) Å] is significantly shorter than that

found in 2 [1.569(3) Å].^8bTo explore more rigorously the

bonding situation in 5, DFT calculations were carried

out for the cation [(η⁵-C₅Me₅)Fe(CO)₂(BMes)]⁺at the

BLYP/TZP level of theory using methods previously

described. The agreement between (fully optimized)

calculated [Fe(1)-B(1) 1.843 Å, B(1)-C(1) 1.495 Å, Fe-

(1)-B(1)-C(1) ) 177.8°, torsion ) 81.6°] and measured

geometric parameters is generally very good, with a

2-3% overestimate in the Fe-B bond length mirroring

previous studies.^23,33On the basis of a population

analysis of the molecular orbitals at the DFT relaxed

F igu r e 2. Structure of one of the two crystallographically

independent cations in the asymmetric unit of [(η⁵-C₅Me₅)-

Fe(CO)₂(BMes)][BAr^f₄], 5. Relevant bond lengths (Å), angles

(deg), and torsion angles (deg): Fe(1)-B(1) 1.792(8), Fe(1)-

C(20) 1.768(7), Fe(1)-(η⁵-C₅Me₅) centroid 1.733(7), B(1)-

C(1) 1.491(10), Fe(1)-B(1)-C(1) 178.3(6), (η⁵-C₅Me₅) cen-

troid-Fe(1)-C(1)-C(2) 91.3(6). Hydrogen atoms are omitted

for clarity.

C₅Me₅) centroid-Fe-N-C torsion angle of ca. 90°

(84.6°) (see Supporting Information).^4b

(ii) Bon d in g Descr ip tion . The solid state structure

of 5 (see Figure 2 and Table 1) is based around an

asymmetric unit containing two (essentially identical)

[(η⁵-C₅Me₅)Fe(CO)₂(BMes)]⁺cations, two [BAr^f]^-an-

4

ions, and a molecule of CH₂Cl₂solvent. There are no

short cation-anion or cation-solvent contacts. Of par-

ticular interest is the near linear Fe-B-C unit [ Fe(1)-

B(1)-C(1) ) 178.3(6)°] and the Fe-B distance [1.792(8)

Å], which is significantly shorter than any transition

metal to boron linkage previously reported. It is also

11% shorter than that found in (η⁵-C₅Me₅)BFe(CO)₄

[2.010(3) Å] (a compound that contains a BfFe donor/

acceptor linkage), 18% shorter than the σ-only single

bond found in the four-coordinate boryl complex (η⁵-C₅-

Me₄Et)Fe(CO)₂BH₂‚PMe₃[2.195(14) Å], and 8.5% shorter

than the shortest Fe-B distance found for any three-

coordinate boryl complex [1.959(6) Å for (η⁵-C₅H₅)Fe(CO)₂-

Bcat].^3a,5d,28,29By means of comparison, the Fe-C

distance in [(η⁵-C₅H₅)Fe(CO)₂(dCCl₂)]⁺[1.808(12) Å] is

ca. 12% shorter than found in typical (η⁵-C₅H₅)Fe(CO)₂

alkyl complexes,^30,31and the Ru-Si distance in [(η⁵-C₅-

Me₅)Ru(PMe₃)₂(dSiMe₂)]⁺[2.238(2) Å] is 6% shorter

than that found in [(η⁵-C₅Me₅)Ru(PMe₃)₂(SiMe₂CH₂-

(28) Yasue, T.; Kawano, Y.; Shimoi, M. Chem. Lett. 2000, 58.

(29) Hartwig, J . F.; Huber, S. J . Am. Chem. Soc. 1993, 115, 4908.

(30) Crespi, A. M.; Shriver, D. F. Organometallics 1985, 4, 1830.

(31) For example, an Fe-C bond length of 2.069(10) Å has been

reported for the complex (η⁵-C₅H₅)Fe(CO)₂(n-C₅H₁₁): Hill, R. O.;

Marais, C. F.; Moss, J . R.; Naidoo, K. J . J . Organomet. Chem. 1999,

587, 28.

(32) Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. J . Am.

Chem. Soc. 1979, 101, 585.

(33) (a) McCullough, E. A., J r.; Apra`, E.; Nichols, J . J . Phys. Chem.

A 1997, 101, 2502. (b) Giju, K. T.; Bickelhaupt, M.; Frenking, G. Inorg.

Chem. 2000, 39, 4776.

2920 Organometallics, Vol. 23, No. 12, 2004

Coombs et al.

(PMe₃)₂(BMes)]⁺shows a complementary lengthening

by 2.3%. Additionally, the BDE predicted for 5 is

significantly greater than that calculated for removal

of the very similar BPh ligand from the most stable

(axial) isomer of the charge neutral complex (OC)₄FeBPh

(110.3 kcal mol^-1).⁵ⁱEnergy decomposition analyses

indicate that this is almost entirely accounted for by a

reduction in the repulsive Pauli term. This in turn is

associated with the more contracted nature of the

frontier orbitals of the metal and borylene fragments

in the cationic complex.^4bFinally, comparison of iso-

electronic FedB and FedC containing systems {e.g., 8

and [(η⁵-C₅Me₅)Fe(CO)₂(CCMe₂)]⁺} reveals a ca. 10 kcal

mol^-1higher BDE for the boron-based ligand.^4bThis

finding is entirely consistent with the stronger σ donor

properties and comparable π acceptor role predicted for

the BNR₂ligand by previous studies.^5a

F igu r e 3. DFT (BLYP/TZP) calculated HOMO-3 for the

cation [(η⁵-C₅Me₅)Fe(CO)₂(BMes)]⁺showing Fe-B π bond-

ing character.

Previous computational studies have predicted sig-

nificant boron-centered reactivity toward nucleophiles

for terminal borylene complexes due to the high positive

charge and LUMO amplitude at boron.^5bOn the other

hand, the current study shows that in the presence of

suitable steric shielding, even complexes bearing a net

positive charge can be isolated. Indeed, it seems likely

that the combination of charge and steric shielding in

5 retards the dimerization process observed for the

putative isolectronic manganese system [(η⁵-C₅H₄Me)-

Mn(CO)₂(BCl)].³⁴The calculated partial charge at boron

(+0.356, Mulliken method) in 5 is certainly consistent

with a highly electrophilic boron center, although the

composition of the LUMO (29.3% Fe 3d_{x -y}, 4.6% Fe

3d_xz, 4.4% Fe 3d_yz, 4.2% B 2p_x) suggests that attack by

nucleophiles at the iron center may also be feasible.

(iii) Rea ctivity. The reactivity of cationic borylene

complex 5 toward a number of different types of reagent

has been investigated, with the aim of elucidating

fundamental patterns of reactivity for this class of

compound. Although some reports of substitution chem-

istry for base-stabilized borylenes^3g,hand of elegant

metal-to-metal borylene transfer chemistry have ap-

peared in the recent literature,^3c,e,fthe fundamental

reactivity of group 13 diyl complexes remains a largely

uncharted area. A number of extensive theoretical

studies have predicted thermodynamically strong M-E

bonds for diyl complexes,⁵but point to probable kinetic

lability stemming from the buildup of positive charge

at the group 13 element on coordination.^5bWith this in

mind we have examined the reactivity of 5 toward a

range of anionic and neutral nucleophiles, with the aim

of exploiting the electrophilicity of the boron center

toward the controlled synthesis of complexes containing

new B-X bonds. In addition, given the similarities

between cationic borylene complexes such as 5 and

silylene and Fischer carbene complexes of the type

[L_nMdEX₂]⁺(E ) Si, C) we were interested in examin-

ing the reactivity of 5 toward unsaturated substrates.

Of particular interest, given the known highly indis-

criminate reactivity of the “free” borylene fragment, is

the development of controlled metal-mediated reactivity

toward organic or organometallic substrates.

F igu r e 4. DFT (BLYP/TZP) calculated HOMO-9 for the

cation [(η⁵-C₅Me₅)Fe(CO)₂(BMes)]⁺showing C-B π bonding

character.

geometry, the Fe-B bond has a 62:38 σ:π breakdown

of the covalent contribution to bonding {cf. 64:36 for [(η⁵-

C₅H₅)Fe(CO)₂(dCH₂)]⁺}. For the lowest energy confor-

mation of the complex 5, examination of the relevant

molecular orbitals (Figures 3 and 4) reveals that the

Fe-B bond is characterized by (i) a deep lying orbital

(HOMO-7, -10.56 eV) with considerable Fe-B σ bond-

2

ing character (21.4% Fe 3d_z, 12.8% B 2p_z) and (ii) a π

type interaction predominantly featuring the Fe 3d_xz

and B 2p_xorbitals. Both the HOMO-3 (-9.21 eV) and

the HOMO-6 (-9.85 eV) feature significant in-phase

contributions from these atomic orbitals (HOMO-3:

17.6% Fe 3d_xz, 4.9% B 2p_x; HOMO-6: 18.1% Fe 3d_xz,

3.1% B 2p_x). The LUMO+3 is the corresponding orbital

of predominantly π* character (10.4% Fe 3d_xz, 35.4% B

2p_x). In addition it is possible to identify molecular

orbitals [chiefly the HOMO-9 (-10.98 eV) and HOMO-2

(-8.91 eV)] that possess C-B π bonding character

characterized by in-phase contributions from the 2p_y

atomic orbitals of the boron and ipso carbon atoms

which lie perpendicular to the Fe-B π bond. These

calculations therefore support a bonding model in which

boron engages in π bonding to both [(η⁵-C₅Me₅)Fe(CO)₂]⁺

and Mes moieties.

A substantial bond dissociation energy (BDE, D_o) of

147.5 kcal mol^-1has been calculated for the removal of

the mesitylborylene ligand from 5. This figure is el-

evated slightly on replacement of the ancillary carbonyl

ligands by trimethylphosphines {e.g., 153.0 kcal mol^-1

for model compound [(η⁵-C₅H₅)Fe(PMe₃)₂(BMes)]⁺} but

is significantly smaller for the removal of BNMe₂from

[(η⁵-C₅Me₅)Fe(CO)₂(BNMe₂)]⁺(8) (125.9 kcal mol^-1).

These changes are primarily due to differences in the

orbital (covalent) contribution to the Fe-B bond; these

in turn reflect the expected trends in the σ and π

properties of the BX ligand and the electronic properties

of the metal center.^4bConsistent with this, the calcu-

lated Fe-B distances for [(η⁵-C₅H₅)Fe(PMe₃)₂(BMes)]⁺

and 8 are, respectively, 2.4% shorter and 1.4% longer

than in 5, and the B-C_ipsodistance in [(η⁵-C₅H₅)Fe-

(a ) Rea ctivity tow a r d Nu cleop h iles. The likeli-

hood of nucleophilic attack at the boron center in 5,

(34) Braunschweig, H.; Colling, M.; Hu, C.; Radacki, K. Angew.

Chem., Int. Ed. 2002, 41, 1359.

FedB Double Bonds

Organometallics, Vol. 23, No. 12, 2004 2921

Sch em e 6. Rea ctivity of Ca tion ic Bor ylen e 5

tow a r d Ha lid e An ion s^a

Fe(1)-B(1)-F(1) ) 37.6(2)°] allow for the presence of

an intramolecular C-H‚‚‚F hydrogen bond incorporat-

ing the fluorine substituent and one of the methyl

groups of the (η⁵-C₅Me₅) ligand (Figure 6). The H‚‚‚F

and C‚‚‚F distances [2.44 and 3.054(4) Å] and the

C-H‚‚‚F angle (120.4°) fall within the ranges expected

for such an interaction.³⁵The ability of a relatively weak

C-H‚‚‚X hydrogen bond to influence conformational

geometry in organometallic boryl systems finds prece-

dent in the solid state structure of tetrachlorocatechol-

boryl complex (η⁵-C₅H₄Me)Fe(CO)₂BO₂C₆Cl₄and pro-

vides further experimental evidence for the relatively

low barrier to rotation about the Fe-B bond that has

been predicted by DFT.^23,36It should be noted that there

is no evidence on the basis of spectroscopic data for the

existence of an intramolecular hydrogen bond in solu-

tion. The packing of 12 in the solid state is also

influenced by π stacking, this interaction being char-

acterized by interplanar distances (ca. 3.5 Å) that are

similar to those reported for analogous systems.³⁷

a

Reaction conditions: (i) halide source ([PPN]Cl, [Ph₄P]Br,

or [ⁿBu₄N]I) 1 equiv, dichloromethane, 18 °C, 1.5-2 h.

widely predicted for analogous charge neutral systems,

would appear to be increased by the net positive charge

and a calculated Mulliken charge at boron of +0.356.

We therefore set out, in the first instance, to examine

its reactivity toward a range of anionic nucleophiles. The

reaction of 5 with convenient soluble sources of halide

anions in dichloromethane leads, as expected, to nu-

cleophilic addition at the boron center (Scheme 6). The

chloro-, bromo-, and iodoboryl complexes are formed

very rapidly (as judged by the ¹¹B NMR spectrum of the

reaction mixture) and can be isolated in reasonable

yields (60-80%). In the case of the reaction with [Ph₄P]-

Br, the known (structurally characterized) precursor

bromoboryl complex 2 is formed;^8breaction with [PPN]-

Cl or [ⁿBu₄N]I leads to the isolation of the new boryl

complexes 10 and 11, together with the appropriate

[BAr^f]^-salt (e.g., [PPN][BAr^f] in the case of 10).

The synthesis and characterization of the complete

series of haloboryl complexes (η⁵-C₅Me₅)Fe(CO)₂B-

(Mes)X [X ) F (12), Cl (10), Br (2), I (11)] offers a unique

opportunity to investigate the electronic influence of the

boron-bound substituents in organometallic boryl com-

plexes. Relevant spectroscopic and structural data for

the four complexes are included in Table 2.

The most obvious structural trend is the sequential

increase in the Fe-B distance on going from bromoboryl

complex 2 to its chloro- and fluoro-substituted analogues

10 and 12. Such a trend can be rationalized in terms of

the expected reduction of both σ donor and π acceptor

properties along the series of ligands -B(Mes)Br,

-B(Mes)Cl, and -B(Mes)F. Hence complex 12 features

easily the longest Fe-B bond (with presumably the

smallest absolute π back-bonding component), a factor

that allows for greater rotational freedom by reducing

steric interactions between the bulky metal- and boryl-

bound substituents. This then helps explain the mark-

edly different conformation adopted by the fluoroboryl

complex and the presence of an intramolecular C-H‚‚‚F

hydrogen bond in the solid state.

4

Furthermore, a similar type of halide transfer reactivity

is observed with [ⁿBu₄N][BF₄]. Abstraction of fluoride

from the tetrafluoroborate anion leads to the formation

of fluoroboryl complex 12 in good yield (Scheme 7).

Thus the chemistry outlined in Schemes 6 and 7

represents a new synthetic route to asymmetric boryl

complexes, and one which gives access to the complete

range of halide-substituted complexes (η⁵-C₅Me₅)Fe-

(CO)₂B(Mes)X (X ) F, Cl, Br, I). Spectroscopic data for

the new compounds 10-12 are in agreement with the

proposed formulations, and in the cases of 10 and 12,

these inferences have been confirmed by the results of

a single-crystal X-ray diffraction study (see Figures 5

and 6 and Table 1).

Carbonyl stretching frequencies for these complexes

decrease from values for iodoboryl 11 (2005 and 1955

cm^-1), which are similar to those of corresponding alkyl

compounds,³⁸to values that are somewhat lower (e.g.,

1989 and 1931 cm^-1for fluoroboryl 9). Such a trend

mirrors the expected decrease in the π acceptor proper-

ties of the boryl ligand on going from -B(Mes)I to

-B(Mes)F, this in turn being determined by competing

XfB π donation from the halide. ¹¹B NMR shifts are

also known to be strongly influenced by the π donor

The molecular structure of chloroboryl complex (η⁵-

C₅Me₅)Fe(CO)₂B(Mes)Cl (10) has much in common with

that reported previously for bromoboryl analogue 2 and

for most other complexes of the type (η⁵-C₅R₅)Fe(CO)₂B-

(Mes)X (X ) halide, alkoxide, aryloxide, etc.).^8b,11As

such, the plane defined by the boryl ligand [i.e., by

atoms B(1), C(1), and Cl(1)] is essentially coplanar with

that defined by the cyclopentadienyl centroid, Fe(1) and

B(1) [ centroid-Fe(1)-B(1)-C1(1) ) 179.5(2)°] and

nearly perpendicular to that defined by the mesityl

fragment [ Cl(1)-B(1)-C(1)-C(8) ) 89.5(2)°]. This

ligand orientation seems to be influenced to a large

degree by minimizing steric interactions between the

bulky mesityl and pentamethylcyclopentadienyl sub-

stituents. The molecular structure of fluoroboryl com-

plex 12, however, offers an instructive contrast with

those of related compounds. A significantly longer Fe-B

distance [2.017(3) vs 1.985(2) Å for 10] and a markedly

different orientation of the boryl ligand [ centroid-

(35) See, for example: (a) Desiraju, G. R. Acc. Chem. Res. 1991, 24,

290. (b) J effrey, G. A. J . Mol. Struct. 1999, 485-486, 293. (c) Vargas,

R.; Garza, J .; Dixon, D. A.; Hay, B. P. J . Am. Chem. Soc. 2000, 122,

4750.

(36) Aldridge, S.; Calder, R. J .; Baghurst, R. E.; Light, M. E.;

Hursthouse, M. B. J . Organomet. Chem. 2002, 694, 9.

(37) See, for example: Hunter, C. A.; Sanders, J . K. M. J . Am. Chem.

Soc. 1990, 112, 5525.

(38) Carbonyl stretching frequencies for alkyl compounds of the type

(η⁵-C₅Me₅)Fe(CO)₂R typically fall in the ranges 2005-1999 and 1944-

1938 cm^-1: DeLuca, N.; Wojcicki, A. J . Organomet. Chem. 1980, 193,

359.

2922 Organometallics, Vol. 23, No. 12, 2004

Coombs et al.

Sch em e 7. Con tr a stin g Rea ctivity of Ca tion ic Bor ylen e 5 tow a r d th e Bor a te An ion s [BP h ₄]^-a n d [BF ₄]^-a

a

Reaction conditions: (i) [PPN][BPh₄] 1 equiv, dichloromethane, 18 °C, 1 h; (ii) [ⁿBu₄N][BF₄] 1 equiv, dichloromethane, 18 °C,

2 h.

Ta ble 2. Com p a r ison of Str u ctu r a l a n d

Sp ectr oscop ic Da ta for Ha lobor yl Com p lexes of

th e Typ e (η⁵-C₅Me₅)F e(CO)₂B(Mes)X

11

2

10

12

X in -B(Mes)X

ν(CO)/cm^-1

d(Fe-B)/Å

δ_B/ppm

I

Br

Cl

F

2005, 1955 1999, 1942 1996, 1937 1989, 1931

-

110.7

1.972(2)

113.0

1.985(2)

112.1

2.017(3)

90.4

π donor properties of fluoride. Very similar trends have

been observed previously, for example in the reported

resonances of the phenylboron dihalides PhBX₂[δ_B

23.8-24.5, 54.1-55.9, 56.1-60.6, and 44.6-48.2 for X

) F, Cl, Br, and I, respectively⁴⁰].

F igu r e 5. Molecular structure of (η⁵-C₅Me₅)Fe(CO)₂B-

(Mes)Cl, 10. Relevant bond lengths (Å), angles (deg), and

torsion angles (deg): Fe(1)-B(1) 1.985(2), B(1)-Cl(1)

1.834(2), B(1)-C(1) 1.569(2), Fe(1)-(η⁵-C₅Me₅) centroid

1.748(2), (η⁵-C₅Me₅) centroid-Fe(1)-B(1)-Cl(1) 179.5(2),

Cl(1)-B(1)-C(1)-C(8) 89.5(2). Hydrogen atoms are omit-

ted for clarity.

In the case of the reaction of 5 with [ⁿBu₄N]I, periodic

monitoring of reaction mixture by IR spectroscopy

reveals that a second, minor product is also formed

(albeit in low yield). This product is present even at

short reaction times (ca. 30 min), and its relative

concentration (with respect to that of the major product,

iodoboryl 11) does not appear to vary significantly with

time. This second compound gives rise to two carbonyl

stretching bands (2017 and 1969 cm^-1) which are in

agreement both with the values reported previously by

Akita, Morooka, and co-workers for (η⁵-C₅Me₅)Fe(CO)₂I

in CH₂Cl₂(2019 and 1971 cm^{-1 18}

and with those

)

measured for a sample of (η⁵-C₅Me₅)Fe(CO)₂I prepared

independently from [(η⁵-C₅Me₅)Fe(CO)₂]Na and I₂(2017

and 1971 cm^-1). Additional confirmation of the identity

of this compound was obtained crystallographically; the

less soluble (η⁵-C₅Me₅)Fe(CO)₂I can be separated from

11 by fractional crystallization from a hexane solution.

Although there are minor differences from the literature

data, the crystal structure obtained (and which is

reproduced in the Supporting Information) unambigu-

ously identifies the minor product as (η⁵-C₅Me₅)Fe(CO)₂I

(13).¹⁸Given the patterns of reactivity that emerge for

5 toward neutral two-electron-donor ligands (vide infra),

together with the seemingly invariant relative concen-

trations of 11 and 13 in the reaction mixture, it seems

likely that 13 is formed by competing attack of iodide

at the iron center of 5, rather than by decomposition of

boryl complex 11. Although nucleophilic attack at iron

formally involves reaction at an 18-electron metal

center, an interchange type mechanism involving con-

certed loss (and scavenging) of the BMes ligand is

implied by kinetic data for other reactions of this type

(vide infra).

F igu r e 6. Molecular structure of (η⁵-C₅Me₅)Fe(CO)₂B-

(Mes)F, 12, showing intramolecular C-H‚‚‚F hydrogen

bonding. Relevant bond lengths (Å), angles (deg), and

torsion angles (deg): Fe(1)-B(1) 2.017(3), B(1)-F(1) 1.350(3),

B(1)-C(3) 1.581(4), Fe(1)-(η⁵-C₅Me₅) centroid 1.736(3),

H(21b)-F(1) 2.44, C(21)‚‚‚F(1) 3.054(4), C(21)-H(21b)-

F(1) 120.4, (η⁵-C₅Me₅) centroid-Fe(1)-B(1)-F(1) 37.6(2),

F(1)-B(1)-C(3)-C(8) 70.7(2). Hydrogen atoms except those

attached to C(21) are omitted for clarity.

properties of boron-bound substituents;³⁹the strongly

upfield resonance measured for fluoroboryl complex 12

compared to 2, 10, and 11 therefore reflects the known

(39) For a discussion of the influence of π donor substituents on ¹¹B

chemical shifts in boryl systems see, for example: 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. In the

case of boron-bound iodine substituents, relativistic effects can also

be important: Kaupp, M.; Malkina, O. L.; Malkin, V. G. Chem. Phys.

Lett. 1997, 265, 55.

The notion of the iron center as a competing site of

electrophilicity is given further impetus by examination

(40) No¨th, H.; Wrackmeyer B. In NMR Basic Principles and

Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag:

Berlin, 1978; Vol. 14.

FedB Double Bonds

Organometallics, Vol. 23, No. 12, 2004 2923

of the reaction of 5 with [BPh₄]^-(Scheme 7). Thus the

Sch em e 8. Disp la cem en t of th e Mesitylbor ylen e

Liga n d fr om 5 by Neu tr a l Tw o-Electr on -Don or

Liga n d s^a

reaction of [(η⁵-C₅Me₅)Fe(CO)₂(BMes)][BAr^f] with a

4

single equivalent of the soluble [PPN]⁺salt of [BPh₄]^-

in dichloromethane proceeds very rapidly (as judged by

¹¹B NMR) to the formation of BPh₃and (η⁵-C₅Me₅)Fe-

(CO)₂Ph (6). Reaction of the tetraphenylborate counter-

ion with an electrophilic metal center to generate BPh₃

finds precedent, for example, in the chemistry of cationic

vanadocene complexes.⁴¹In monitoring the reaction of

5 with [BPh₄]^-by ¹¹B NMR spectroscopy, no evidence

was obtained for any competing reaction involving

nucleophilic transfer of the Ph^-moiety to the boron

center. It might reasonably be expected that the product

of such a reaction, (η⁵-C₅Me₅)Fe(CO)₂B(Mes)Ph, might

give rise to a ¹¹B resonance similar to that reported

previously by Hartwig for the diphenylboryl complex (η⁵-

C₅H₅)Fe(CO)₂BPh₂(δ_B121).²⁹In the event no signals

in this region of the spectrum were observed at any

time. Although it is not possible to rule out conclusively

a mechanism involving initial transfer of Ph^-to the

boron center followed by rapid decomposition of the

boryl complex, (η⁵-C₅Me₅)Fe(CO)₂B(Mes)Ph, so formed,

such a pathway would appear unlikely because (i)

thermolysis of the related compound (η⁵-C₅H₅)Fe(CO)₂-

BPh₂proceeds rapidly only at elevated temperatures

(75-95 °C)^29,42and (ii) decomposition of (η⁵-C₅H₅)Fe(CO)₂-

BPh₂under either thermal or photochemical conditions

has been reported to generate (in addition to BPh₃) the

dimer [(η⁵-C₅H₅)Fe(CO)₂]₂rather than (η⁵-C₅H₅)Fe(CO)₂-

Ph.²⁹

a

Reaction conditions: (i) excess ligand (CO, H₂CdCH^tBu,

or OdCPh₂), dichloromethane, 18 °C, 2-16 h.

NMR), but in this case results in the formation of a

single metal-containing species. Filtration of the reac-

tion mixture, removal of volatile components in vacuo,

and recrystallization from a dichloromethane/hexane

mixture at -30 °C leads to the isolation of [(η⁵-C₅Me₅)-

Fe(CO)₃][BAr^f] (14) as a red crystalline material in ca.

4

75% yield. 14 has been characterized by IR and multi-

nuclear NMR spectroscopies and by mass spectrometry,

with unambiguous confirmation being obtained by

single-crystal X-ray diffraction (see Table 1 and Sup-

porting Information). Structural and spectroscopic data

for the [(η⁵-C₅Me₅)Fe(CO)₃]⁺cation in 14 are very

similar to those reported previously by McArdle for the

corresponding [BF₄]^-salt.^27b

The isolation of 14 as the metal-containing product

from the reaction of 5 with CO implies that displace-

ment of the BMes ligand from the coordination sphere

of iron is not limited to its reactions with anionic

nucleophiles such as [BPh₄]^-or I^-(see Scheme 8).

Similar reactivity has been reported by Tilley for

cationic ruthenium silylene complexes toward ligands

containing nitrogen or phosphorus donor atoms. Thus

reaction of [(η⁵-C₅Me₅)Ru(PMe₃)₂(SiMe₂)]⁺with PMe₃or

PPh₃ultimately leads to [(η⁵-C₅Me₅)Ru(PMe₃)₂(PR₃)]⁺

(R ) Me, Ph) through decomposition of the initially

formed Si-ligated complex [(η⁵-C₅Me₅)Ru(PMe₃)₂(SiMe₂-

PR₃)]⁺.^2eAlthough no analogous intermediate species

could be definitively identified in the reaction of 5 with

either PPh₃or CO, it is not possible to rule out an

overall substitution pathway involving initial formation

of a labile B-ligated complex.

(b) Rea ctivity tow a r d Un sa tu r a ted Su bstr a tes.

The versatile and widely exploited reactivity toward

unsaturated substrates of organometallic complexes

containing MdC double bonds,¹together with the obvi-

ous similarities between electrophilic Fischer carbene

and borylene complexes, prompted us to examine the

reactivity of 5 toward simple unsaturated species con-

taining CdC, CtC, or CdO bonds. However these

studies reveal that the predominant mode of reactivity

of 5 toward compounds of this type does not involve the

cyclopropanation or metathesis chemistry typical of

MdC bonds,^1,44but rather involves borylene ligand

displacement, even by substrates that are typically

thought of as forming weak metal-ligand bonds.

Reaction of 5 with 3,3-dimethyl-1-butene over a period

of 16 h leads to complete consumption of the starting

The experimental observation of alternative sites of

reactivity toward nucleophiles finds ample precedent,

for example in the chemistry of closely related carbonyl

complexes. Thus displacement of CO from the metal

center in [(η⁵-C₅R₅)Fe(CO)(L)₂]⁺by an incoming two-

electron donor is a textbook reaction, with alternative

carbon-centered reactivity being observed, for example,

in the reaction with hydroxide to generate metallocar-

boxylates such as [(η⁵-C₅R₅)Fe(CO)(PPh₃)(CO₂)]^-.⁴³What

is particularly intriguing in the case of 5 is that

reactivity toward the borate anions [BPh₄]^-and [BF₄]^-

should apparently lead to highly selective, but opposing

sites of attack. Given the calculated partial charges at

boron and iron (+0.356 and -0.338, respectively) and

the composition of the LUMO (38.3% total from Fe

2

3d_{x -y}, 3d_xz, and 3d_yz; 4.2% from B 2p_x), an attractive

(if simplistic) interpretation is that attack by the harder

nucleophile [BF₄]^-is charge driven (toward boron),

while that by the softer nucleophile [BPh₄]^-is influ-

enced by the predominant localization of the LUMO at

iron.

The reactivity of 5 toward archetypal neutral two-

electron ligands such as phosphines and CO has also

been examined. In the case of tertiary phosphines

(exemplified by PPh₃) an oily mixture of products was

obtained on addition of 1 equiv of the ligand to a solution

of 5 in dichloromethane. The analogous reaction of 5

with CO over a period of 6 h also proceeds with complete

consumption of the starting material (as judged by ¹¹B

(41) Choukroun, R.; Douziech, B.; Pan, C.; Dahan, F.; Cassoux, P.

Organometallics 1995, 14, 4471.

(42) Additionally, in our experience boryl complexes of the type (η⁵-

C₅R₅)Fe(CO)₂B(Ar)X tend if anything to be thermally more stable for

R ) Me than R ) H (see ref 8b).

(44) 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.

(43) See, for example: Gibson, D. H.; Ong, T.-S.; Ye, M. Organo-

metallics 1991, 10, 1811.

2924 Organometallics, Vol. 23, No. 12, 2004

Coombs et al.

material, as judged by ¹¹B NMR. The metal-containing

product isolated from dichloromethane solution displays

¹¹B and ¹⁹F NMR resonances indicative of the [BAr^f]^-

4

anion, and the H and ¹³C NMR spectra are consistent

1

with the presence of (η⁵-C₅Me₅), CO, and coordinated

alkene moieties. In particular, the ¹H resonances for the

three alkene protons of the ^tBu(H)CdCH₂unit are

shifted significantly upfield (δ_H2.68, 3.11, 3.99) com-

pared to the free alkene (δ_H4.82, 4.92, 5.85), as expected

for coordination to the iron center. Similar upfield shifts

are observed for the alkene moiety on coordination to

the [(η⁵-C₅Me₅)Fe(CO)₂]⁺fragment for ethylene and

styrene (e.g., δ_H5.24, 5.75, 6.72 to 3.03, 3.97, 4.89 for

styrene).⁴⁵In addition the ES+ mass spectrum displays

prominent features corresponding to the molecular ion

[(η⁵-C₅Me₅)Fe(CO)₂(H₂CdCH^tBu)]⁺(M⁺, m/z 331, 50%)

and to fragment ions formed by loss of either alkene or

carbonyl ligands ([M - alkene]⁺, m/z 247, 40%; [M -

2CO]⁺, m/z 275, 100%). Carbonyl stretching frequencies

of 2033 and 1997 cm^-1are slightly lower than those

reported for [(η⁵-C₅Me₅)Fe(CO)₂(H₂CdCHR)][PF₆] (R )

H, 2050, 2020 cm^-1; R ) Ph, 2060, 2020 cm^{-1 45}), for

which an η²mode of alkene coordination might be

expected by comparison with the structurally authen-

ticated ruthenium styrene complex,⁴⁵but are similar to

those reported for the crystallographically characterized

complex [(η⁵-C₅H₅)Fe(CO)₂(η²-bicyclo[3.2.2]non-1-ene)]-

[CF₃SO₃] (2030, 2010 cm^-1).⁴⁶Thus the available spec-

troscopic data point to a cationic complex featuring an

η²-coordinated alkene fragment, i.e., [(η⁵-C₅Me₅)Fe(CO)₂-

F igu r e 7. Molecular structure of the cationic component

of [(η⁵-C₅Me₅)Fe(CO)₂(η¹-OCPh₂)][BAr^f], 16. Relevant bond

4

lengths (Å), angles (deg), and torsion angles (deg): Fe(1)-

C(1) 1.801(5), Fe(1)-C(2) 1.793(5), Fe(1)-O(3) 1.982(3),

Fe(1)-(η⁵-C₅Me₅) centroid 1.723(4), O(3)-C(13) 1.256(5),

Fe(1)-O(3)-C(13) 136.6(3), (η⁵-C₅Me₅) centroid-Fe(1)-

O(3)-C(13) 141.3(4). Hydrogen atoms are omitted for

clarity.

2030 cm^-1).⁴⁹Although no data were reported for the

CdO stretch of the coordinated benzophenone ligand for

this compound, the measured value for 16 (1612 cm^-1

)

is very similar to that reported for the structurally

characterized yttrium complex (η⁵-C₅Me₅)₂Y(OdCPh₂)-

Cl (1614 cm^-1), which contains an O-bound η¹-OdCPh₂

ligand.⁵⁰The crystal structure of 16 (Figure 7 and Table

1) confirms this mode of attachment for the benzophe-

none ligand; the Fe-O distance and Fe-O-C angle

[1.982(3) Å and 136.6(3)°] are very similar to those

reported previously for cationic η¹-ketone complexes

{e.g., 1.983(5)° and 136.9(4)°, respectively, for [(η⁵-C₅H₅)-

Fe(CO)₂(OdCC₆H₆)][BF₄]}, and the lengthening of the

benzophenone CdO bond [1.256(5) Å in 16] compared

to the free ligand (1.23 Å) mirrors that seen in Tb[N-

(SiMe₃)₂]₃(η¹-OdCPh₂) [1.264(14) Å].^51,52Although a

number of ketone and aldehyde complexes of iron have

been reported, complex 16 represents the first structur-

ally characterized iron-containing benzophenone com-

plex.

Within the context of the current study the conversion

of 5 to 16 in particular appears somewhat surprising,

especially given that the charge neutral chromium

aminoborylene complex (OC)₅CrBN(SiMe₃)₂can be re-

covered unchanged from acetone solution.⁵³Strong

binding of group 13 diyl ligands to transition metal

centers has been predicted by a number of recent

computational studies,⁵and a bond dissociation energy

of 147.5 kcal mol^-1has been predicted for the mesityl-

borylene ligand in [(η⁵-C₅Me₅)Fe(CO)₂(BMes)]⁺.^4bThat

the reactivity of 5 toward a number of neutral two-

electron-donor ligands (including relatively weak donors

such as ketones) is undoubtedly characterized by sub-

(η²-H₂CdCH^tBu)][BAr^f] (15), an inference that was

4

subsequently confirmed crystallographically. The struc-

ture of 15 (see Table 1 and Supporting Information)

features a C-Fe-C angle [37.4(2)°] and Fe-C [2.096(6)

and 2.233(7) Å] and C-C distances [1.393(9) Å] for the

coordinated alkene ligand that are consistent with

previous examples of η²coordination to a [(η⁵-C₅R₅)Fe-

(CO)₂]⁺fragment.^46-48Additionally the angles sub-

tended at the iron center by the carbonyl ligands and

by the alkene CdC centroid [92.6(3)-96.8(3)°] are

consistent with a three-legged piano stool geometry at

the metal.

The reactivity of 5 toward more polar multiple bonds

was also examined, with a similar outcome being

observed. Thus, reaction with benzophenone in dichlo-

romethane over a 3 h period leads to complete consump-

tion of the starting material; recrystallization from

dichloromethane/hexane generates the dark red benzo-

phenone complex [(η⁵-C₅Me₅)Fe(CO)₂(η¹-OdCPh₂)][BAr^f₄]

(16) as the metal-containing product in 55-60% yield.

The identity of 16 was established by multinuclear

NMR, IR, and mass spectrometry and confirmed crys-

tallographically. In particular the stretching frequencies

for the CO ligands (2050, 2004 cm^-1) show the expected

shift to lower wavenumber in comparison with those

reported for the analogous (η⁵-C₅H₅) complex (2078,

(49) Schmidt, E. K. G.; Theil, C. H. J . Organomet. Chem. 1981, 209,

373.

(50) Evans, W. J .; Fujimoto, C. H.; J ohnston, M. A.; Ziller, J . W.

Organometallics 2002, 21, 1825.

(51) Boudjouk, P.; Woell, J . B.; Radonovich, L. J .; Eyring, M. C.

Organometallics 1982, 1, 582.

(52) Allen, M. Aspinall, H. C.; Moore, S. R.; Hursthouse, M. B.;

Karvalov, A. I. Polyhedron 1992, 11, 409.

(45) Guerchais, V.; Lapinte, C.; The´pot, J .-Y.; Toupet, L. Organo-

metallics 1988, 7 604.

(46) Bly, S. R.; Bly, R. K.; Hossain, M. M.; Lebioda, L.; Raja, M. J .

Am. Chem. Soc. 1988, 110, 7723.

(47) Hu, Y.-R.; Leung, T. W.; Chin, S.-S.; Wojcicki, A.; Calligaris,

M.; Nardin, G. Organometallics 1985, 4, 1001.

(48) Turnbull, M. M.; Foxman, B. M.; Rosenblum, M. Organome-

tallics 1988, 7, 200.

(53) Braunschweig, H. Personal communication.

FedB Double Bonds

Organometallics, Vol. 23, No. 12, 2004 2925

stitution of the mesitylborylene ligand would therefore

appear to be thermodynamically counterintuitive. On

the other hand, extrusion of the MesB fragment from

the coordination sphere of a cationic group 8 metal

center has some precedent in silylene chemistry, with

[(η⁵-C₅Me₅)Ru(PMe₃)₂(SiMe₂)]⁺ultimately undergoing

substitution at ruthenium with tertiary phosphines.

[(η⁵-C₅Me₅)Ru(PMe₃)₂(PR₃)]⁺(R ) Me or Ph) and oli-

gomeric (Me₂Si)_nare the final products identified in this

case.^2eA similar irreversible extrusion of the MesB

fragment from 5 is conceivable, although given the

observation of a common boron-containing product from

all of the displacement reactions described above,

together with the known (high) reactivity of “free”

borylenes toward hydrocarbons, a more likely possibility

is irreversible reaction of the BMes fragment with the

common dichloromethane solvent. A number of experi-

mental observations add weight to this hypothesis.

Although complete characterization of the common

boron-containing product was prevented by its high

volatility and extreme moisture sensitivity, its measured

¹¹B chemical shift (δ_B17.0) is consistent with a species

such as [Mes(Cl₂CH)B(µ-H)]₂formed by dimerization of

the initial product of BMes insertion into one of the

C-H bonds of dichloromethane (cf. chemical shifts of

δ_B17.5 and 25.9 for B₂H₆and (MesBH₂)₂, respec-

tively^40,54).⁵⁵Additionally, ¹H signals consistent with

mesityl and BH protons show similar concentration/time

profiles, and rapid insertion reactions of “free” borylenes

into C-H and C-C bonds have previously been reported

in a number of studies.⁵⁶Of particular relevance is the

intramolecular C-H insertion reaction undergone by

the putative borylene (Me₃Si)₃CB to generate the hy-

dride-bridged dimer [(Me₃Si)₂C(Me₂Si)CH₂B(µ-H)]₂.⁵⁷

Although other insertion reactions of the MesB frag-

ment (e.g., into the C-Cl bond of dichloromethane) can

be envisaged, we believe that kinetic data in particular

are most consistent with a C-H insertion process (vide

infra).

F igu r e 8. Plots of ln([5]/[5]₀) versus time (s) for the

thermolysis of cationic borylene 5 in deuterio ([) and

normal dichloromethane (b). The linear plots (R²) 0.994

and 0.988, respectively) yield values of 1.52 × 10^-5and 5.78

× 10^-5s^-1for the first-order rate constants k_Dand k_H,

repectively.

unlikely a bimolecular decomposition process involving

interaction between two molecules of 5. Furthermore,

comparison of the kinetic data for the reactions in

CD₂Cl₂and CH₂Cl₂reveals a primary kinetic isotope

effect (k_H/k_D) 3.8), an observation consistent not only

with involvement of the dichloromethane solvent in the

rate-determining step of the borylene displacement

reaction but also with a significant degree of C-H bond

breakage in the transition state. It therefore seems

probable that insertion of the BMes fragment into one

of the C-H bonds of the CH₂Cl₂solvent is occurring

during the ligand displacement process. Clearly, given

the high BDE calculated for 5, the observation of facile

ligand displacement at 40 °C must imply significant

involvement of the dichloromethane molecule during the

displacement of the BMes fragment from the metal

coordination sphere. The thermodynamics of the inser-

tion process (i.e., the formation of B-H and B-C bonds)

would then explain the displacement of the BMes ligand

even by relatively poor donors.

An alternative approach to determining the fate of

the boron-containing fragment in ligand displacement

reactions of 5 is the use of trapping reagents which

might be expected to give tractable and readily char-

acterized products on reaction with any mesitylborylene

generated. West has previously reported the trapping

of “free” borylene by an internal alkyne to generate a

stable boracyclopropene, and we initially investigated

the reaction of 5 with similar trapping agents.⁵⁹How-

ever, the course of such reactions using either neat

alkyne [e.g., bis(trimethylsilyl)acetylene] or a solution

in dichloromethane did not appear to be straightfor-

ward, and no spectroscopic evidence was obtained at any

point for the generation of a boracyclopropene, Mes-

BC₂R₂.⁵⁹Tokitoh and co-workers, on the other hand,

have reported the photolytic generation of sterically

encumbered arylborylenes and their trapping with

dialkyldichalcogenides (e.g., Me₂S₂) to generate species

of the type ArB(ER)₂.⁶⁰Hence we investigated the use

of such trapping agents in the chemistry of 5. Accord-

ingly, the reaction of 5 with Me₂S₂in hexane at room

temperature leads to quantitative and very rapid (by

The fact that the same boron-containing product is

generated in each of the borylene displacement reactions

(even with relatively poor donors such as benzophenone)

prompted us to examine whether, under forcing condi-

tions, a similar substitution of the borylene ligand might

be brought about by the dichloromethane solvent alone.⁵⁸

At room temperature 5 is stable in dichloromethane

solution over a period of several days. However at 40

°C, a solution of 5 in CD₂Cl₂decomposes over a period

of ca. 24 h. This decomposition process, monitored by

1

¹¹B and H NMR reveals not only that the same boron-

containing product is formed as in other ligand dis-

placement reactions but that the decomposition process

is first order in [5] (Figure 8). This result renders

(54) Entwhistle, C. D.; Marder, T. B.; Howard, J . A. K.; Fox, M. A.;

Mason, S. A. J . Organomet. Chem. 2003, 680, 165.

(55) The species giving rise to the resonance at δ_B17 does not appear

to be the product of a hydrolysis reaction. Independent hydrolysis of 5

generates (MesBO)₃and MesB(OH)₂, which give rise to ¹¹B NMR

resonances at ca. 30 ppm.

(56) See, for example: Grigsby, W. J .; Power, P. P. J . Am. Chem.

Soc. 1996, 118, 7981, and references therein.

(57) Mennekes, T., Paetzold, P.; Boese, R. Angew. Chem., Int. Ed.

1990, 29, 899.

(58) Dichloromethane complexes of the type [(η⁵-C₅R₅)Fe(CO)₂(CH₂-

Cl₂)]⁺have previously been reported: (a) Reger, D. L.; Coleman, C. J .

Organomet. Chem. 1977, 131, 153. (b) J ohnston, P. Hutchings, G. J .;

Denner, L.; Boeyens, J . C. A.; Coville, N. J . Organometallics 1987, 6,

1292.

(59) (a) Pachaly, B.; West, R. Angew. Chem., Int. Ed. Engl. 1984,

23, 454. (b) Eisch, J . J .; Shafii, B.; Odon, J . D.; Rheingold, A. L. J . Am.

Chem. Soc. 1990, 112, 1847.

(60) Ito, M.; Tokitoh, N.; Kawashima, T.; Okazaki, R. Tetrahedron

Lett. 1999, 40, 5557.

2926 Organometallics, Vol. 23, No. 12, 2004

Coombs et al.

(Ar ) Mes or 2,6-Trp₂C₆H₃) with Na[BAr^f], which lead

Sch em e 9. Rea ctivity of 5 tow a r d Dim eth yl

Disu lfid e: Ch em ica l Tr a p p in g of th e Bor ylen e

F r a gm en t^a

4

to the generation of no detectable borylene products. On

the other hand, the use of alternative halide abstraction

agents containing more reactive anions such as [BPh₄]^-

or [BF₄]^-leads to the isolation of complexes derived

from attack at either the boron or iron center.

Crystallographic, spectroscopic, and DFT studies of

5 are consistent with an FedB double bond comprising

a BfFe σ interaction, supplemented by FefB π back-

bonding into one of the formally vacant p orbitals at

boron. On the basis of a DFT population analysis the

Fe-B bond has a 62:38 σ:π breakdown of the covalent

contribution to bonding compared to a 64:36 breakdown

for archetypal Fischer carbene [(η⁵-C₅H₅)Fe(CO)₂-

(dCH₂)]⁺}.

a

Reaction conditions: (i) Me₂S₂1.5 equiv, hexane, 2 h.

¹¹B NMR) conversion to a single boron-containing spe-

cies, giving rise to a broad signal at δ_B66.0. Isolation

of the oily hexane-soluble product and comparison of

spectroscopic data with those reported previously con-

firm that this product is the bis(methylthio)borane

MesB(SMe)₂(17) (Scheme 9).¹⁹A similar reaction with

Me₂Se₂also generates a single boron-containing prod-

uct, the ¹¹B NMR resonance of which (δ_B75.4) is

appropriate for a bis(methylseleno)aryl borane [e.g., δ_B

76.6 and 70.3 for 2,4,6-(R₂HC)₃C₆H₂B(SeMe)₂(R )

SiMe₃) and PhB(SeMe)₂, respectively].^40,60Chemical

trapping experiments therefore mirror the results of

thermolysis studies in being consistent with rapid

insertion of the ejected BMes ligand into reactive

available linkages.

DFT-based analyses of the charge distribution and

LUMO composition for 5 indicate that both boron and

iron centers may be likely centers for nucleophilic

attack. Such predictions are borne out in practice by an

investigation of the reactivity of 5 toward a number of

different classes of reagent. Thus reactions proceeding

to (η⁵-C₅Me₅)Fe(CO)₂B(Ar)X via nucleophilic addition at

the boron center predominate for the halide ions Cl^-,

Br^-and I^-and for BF₄^-, whereas substitution of the

-

borylene ligand at the iron center is observed for BPh₄

,

CO, H₂CdC(H)^tBu, and Ph₂CdO. Despite unfavorable

ligand binding thermodynamics, the latter chemistry is

thought to be driven by irreversible removal of the MesB

ligand, which can be trapped, for example, by insertion

into the S-S bond of Me₂S₂trapping agent.

Con clu sion s

Application of halide abstraction chemistry to asym-

metric haloboryl complexes of the type (η⁵-C₅Me₅)Fe-

(CO)₂B(ER_n)X leads to the development of the first

synthetic route to cationic multiply bonded group 13 diyl

complexes. Thus, abstraction of bromide from (η⁵-C₅-

Ack n ow led gm en t. We would like to acknowledge

the support of the EPSRC (grants GR/R04669/01 and

GR/S07667/01, and Ph.D. studentship for D.L.C.), the

Royal Society, and Cardiff University for the funding

of this project. The assistance of the EPSRC National

Mass Spectrometry Service Centre, University of Wales

Swansea, and the EPSRC Crystallography Service,

University of Southampton (structure of compound 13),

is also gratefully acknowledged. All calculations were

carried out using the Helix computing facility at Cardiff

University.

Me₅)Fe(CO)₂B(Mes)Br (2) by Na[BAr^f] leads to the

4

isolation of colorless crystalline [(η⁵-C₅Me₅)Fe(CO)₂-

(BMes)][BAr^f] (5) in 50-60% yield, while similar chem-

4

istry applied to (η⁵-C₅Me₅)Fe(CO)₂B(NMe₂)Br (7) at -20

°C leads to the generation of the highly reactive ami-

noborylene [(η⁵-C₅Me₅)Fe(CO)₂(BNMe₂)][BAr^f₄] (8), which

can be characterized in solution by multinuclear NMR

and IR spectroscopies and chemically trapped by chlo-

ride to yield the known compound (η⁵-C₅Me₅)Fe(CO)₂B-

(NMe₂)Cl (9). In the case of arylborylene complexes, both

the steric properties of the putative borylene substituent

(and of the metal fragment) and the inherent coordinat-

ing properties/reactivity of the counterion employed

appear to play key roles in the isolation (or otherwise)

of stable, tractable cationic borylene complexes. Thus

the need for suitable steric shielding of both boron and

metal centers is demonstrated by the reactions of (η⁵-

C₅Me₅)Fe(CO)₂B(Ph)Cl (4) or (η⁵-C₅H₅)Fe(CO)₂B(Ar)Br

Su p p or tin g In for m a tion Ava ila ble: Full details (includ-

ing CIF files) of the crystallographic studies of compounds 5,

10, 12, 14, 15, and 16, together with those for 13 (not reported

in main text); variable-temperature NMR spectra for fluorobo-

ryl complex 12; details of the DFT optimized structures for 5,

8, and [(η⁵-C₅H₅)Fe(PMe₃)₂(BMes)]⁺; and a complete listing of

the molecular orbital energies and compositions for 5. This

material is available free of charge via the Internet at

http://pubs.acs.org.

OM049793E

Article Doi

Fe=B double bonds: Synthetic, structural, and reaction chemistry of cationic terminal borylene complexes

DOI: 10.1021/om049793e

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Article abstract of DOI:10.1021/om049793e

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