Gallium-boron donor-acceptor bonds

Article abstract of DOI:10.1039/b106599h

Examples of compounds with gallium-boron donor-acceptor bonds, HC[MeC(2,6-Prⁱ₂C₆H₃)N] ₂Ga→B(C₆F₅)₃ 3 and (η⁵-C₅Me₅)Ga→B(C₆F

Full text of DOI:10.1039/b106599h

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Gallium–boron donor–acceptor bonds
Ned J. Hardman,a Philip P. Power,*a John D. Gorden,b Charles L. B. Macdonaldb and Alan H.
Cowley*b
a Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California
95616, USA. E-mail: pppower@ucdavis.edu
b Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712,
U.S.A
Received (in Columbia, MO, USA) 19th June 2001, Accepted 1st August 2001
First published as an Advance Article on the web 29th August 2001
Examples of compounds with gallium–boron donor–ac-
orientation of the C6F5 rings relative to the C6H3Pri2-2,6
substituents. The C5Me5 group of 4 is attached to gallium in an
ceptor bonds, HC[MeC(2,6-Pri2C6H3)N]2Ga?B(C6F5)3
3
and (h5-C5Me5)Ga?B(C6F5)3 4 have been prepared by
treatment of the free gallanediyls with B(C6F5)3; the
structures of both compounds were determined by X-ray
crystallography.
h fashion (Fig. 2) and the ring centroid–Ga–B moiety is
5
essentially linear (176.65(6)°). While the average Ga–C
Although most of the organometallic chemistry of gallium
features this element in the +3 oxidation state, the recent
literature reflects an emerging interest in gallium( ) derivatives.
I
Typically, RGa species tend to aggregate into weakly bound
tetrameric1 or hexameric2 entities; however, if a sufficient steric
blockade is deployed, it is possible to isolate monomeric
derivatives.3,4 Thus, for example, (Dipp2nacnac)Ga
1
(HC[MeC(2,6-Pri2C6H3)N]2Ga) is monomeric in the solid
5
state4 while (h -C5Me5)Ga 2 (Cp*Ga) is monomeric in the
vapour state.5 Molecular orbital calculations on 16 and 27
indicate that both molecules adopt a singlet ground state.
Accordingly, 1 and 2 are anticipated to exhibit Lewis base
behaviour due to the presence of a lone pair of electrons on
gallium. Indeed, it has been shown that 2 displays ligative
behaviour towards transition metal moieties8 but such com-
plexes do not exemplify pure Lewis base behaviour because of
the possibility of varying degrees of back donation on the part
of the d-block element. Herein, we disclose two examples of
compounds with gallium–boron donor–acceptor bonds.
Fig. 1 Thermal ellipsoid plot (30%) one of the two crystallographically
independent molecules of 3. H and F atoms are not shown. Selected bond
distances (Å) and angles (°) Ga(1)–B(1) 2.156(1), Ga(2)–B(2) 2.142(3),
Ga–N(av.) 1.942(6), N(1)–C(1) 1.331(4), C(1)–C(2) 1.395(4), C(2)–C(3)
1.388(4), C(3)–N(2) 1.339(4), B–C(av.) 1.641(6); N(1)–Ga(1)–N(2)
96.1(1), N(3)–Ga(2)–N(4) 95.7(1), S°CB(1)C 334.3(2), S°CB(2)C
332.8(2).
The borane complexes HC[MeC(2,6-Pri2C6H3)N]2Ga-
5
?B(C6F5)3 3 and (h -C5Me5)Ga?B(C6F5)3 4 were obtained
by treatment of 1 and 2, respectively, with B(C6F5)3 in toluene
solution.† The 11B NMR spectra of 3 and 4 comprise broad
singlets at d 220.3 and 217.94, respectively, which fall in the
tetracoordinate boron region and the 19F chemical shifts of the
equivalent C6F5 groups are similar to those reported for other
Lewis base complexes of B(C6F5)3.9 In the 1H NMR spectrum
of 3 the b-diketiminate ligand resonances display a symmetric
2
pattern thus implying syn h bonding to gallium, while for 4 the
5
equivalence of the methyl protons is suggestive of h -Cp* ring
attachment. The foregoing spectroscopic indications were
confirmed by means of X-ray crystallography.‡ The crystalline
states of both complexes consist of individual molecules of 3
and 4 and there are no unusually short intermolecular contacts.
In complex 3, which crystallizes as two crystallographically
independent but chemically identical monomeric complexes
(Fig. 1), the gallium atoms have trigonal planar geometry with
an average Ga–N bond distance of 1.942(6) Å. The latter
distance is considerably shorter than the Ga–N bond distances
(av. 2.054(2) Å) in the precursor 1.4 This is consistent with a
decrease in the partial antibonding character of these bonds
upon conversion of the gallium lone pair into a gallium–boron
donor–acceptor bond, and the concomitant development of
positive and negative charges on the gallium and boron atoms,
respectively. The C3N2Ga array of the b-diketiminate ring is
essentially planar with C–C and C–N distances (Fig. 1 legend)
that are indicative of delocalization of the p-electrons. As is
apparent from Fig. 1, the B–Ga–N angles differ slightly (by ca.
5°) which is a result of differing steric interactions caused by the
5
Fig. 2 Thermal ellipsoid plot (30%) of (h -C5Me5)Ga?B(C6F5)3 4 showing
the atom-labeling scheme. Hydrogen and fluorine atoms have been removed
for clarity. Selected bond distances (Å) and angles (°) Ga–B 2.160(2), Ga–
Cp*centroid 1.8648(9), Ga–C(11) 2.217(2), Ga–C(12) 2.226(2), Ga–C(13)
2.227(2), Ga–C(14) 2.238(2), Ga–C(15) 2.230(2), B–C(21) 1.623(3), B–
C(31) 1.629(3), B–C(41) 1.633(3); C(21)–B–C(31) 113.15(15), C(21)–B–
C(41) 115.7(2), C(31)–B–C(41) 113.23(15), S°CBC 342.2(2).
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Chem. Commun., 2001, 1866–1867
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106599h

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through Celite. The filtrate was concentrated in vacuo until the volume was
ca. 10 mL; slow cooling to 220 °C afforded a crop of colourless crystals,
0.65 g, 0.91 mmol, 75% yield; mp 125–135 °C (decomp.). 1H NMR (300.00
MHz, 295 K, C6D6): d 1.465 (s, C5Me5, 15 H); 11B{1H} NMR (96.28 MHz,
295 K, C6D6): d 217.94 (br, w1/2 = 2887 Hz]; 13C{1H} NMR (75.48 MHz,
295 K, C6D6): d 147.62 (d, o-C6F5, 1JCF 239 Hz), 141.36 (d, p-C6F5, 1JCF
254 Hz), 137.62 (d, m-C6F5, 1JCF 252 Hz), 129.28 (s, ipso-C6F5), 114.64 (s,
C5(CH3)5), 8.54 (s, C5(CH3)5); 19F{1H} NMR (282.0 MHz, 295 K, C6D6):
d 2131.28 (s, m-C6F5), 2153.81 (s, p-C6F5), 2163.05 (s, o-C6F5).
‡ Crystal data for 3: monoclinic, space group P21/c, a = 18.1799(15), b =
23.8400(19), c = 21.5818(18) Å, b = 111.279(2)°, V = 8716.0(12) Å3, Z
= 8, Dc 1.523 g cm23, R1 = 0.0431, wR2 = 0.1138. For 4: monoclinic,
space group P21/c, a = 9.2222(18), b = 24.000(5), c = 12.063(2) Å, b =
93.67(3)°, V = 2664.5(9) Å3, Z = 4, Dc = 1.787 g cm23, R1 = 0.0332,
wR2 = 0.0910. Suitable single crystals of 3 and 4 were covered with mineral
oil and mounted on a Bruker Smart AXS 1000 diffractometer at 90 K (3) or
a Nonius-Kappa CCD diffractometer at 133 K (4). Data sets for 3 and 4 were
collected using Mo-Ka radiation (l = 0.71073 Å). For 3, there were a total
of 17 805 independent reflections in the range 1.2 < q < 26.38° and, of
these, 11 235 reflections were observed (I > 2.0s(I)) for the solution (direct
methods) and refinement (full matrix, least squares on F2). For 4, a total of
6009 independent reflections were collected in the range 5.98 < 2q <
distance of 2.228(2) Å is considerably shorter than those
5
5
reported for (h -C5Me5)Ga (2.405(4) Å)5 and [(h -C5Me5)Ga]6
(2.380(9) Å),10 they are in good agreement with those reported
5
for the transition metal derivatives (h -C5Me5)-
5
GaFe(CO)4 (2.226(2) Å) and (h -C5Me5)GaCr(CO)5 (2.260(3)
Å).8 The cause of this Ga–C bond shortening upon coordination
is similar to that described above for the Ga–N bond shortening
of 3. The Ga–B bond distances in 3 (2.142(3) Å, Ga(2)–B(2)
and 2.156(3) Å, Ga(1)–B(1)) and 4 (2.160(2) Å) are slightly
longer than that predicted for a single bond from the sum of the
covalent radii of Ga (1.25 Å) and B (0.85 Å). These bond
distances may be compared with the average Ga–B distances
reported for a variety of gallium-substituted carboranes
(2.14–2.33 Å).11 Due to the donor action of the (Dipp2nac-
5
nac)Ga and (h -C5Me5)Ga fragments, the geometry of the
B(C6F5)3 changes from trigonal planar to distorted tetrahedral.
The extent of the geometrical change from trigonal planar
toward tetrahedral of B(C6F5)3 has been taken to be an
indication of the strength of the donor–acceptor interactions.9
The sums of the C–B–C bond angles at boron in compound 3
(334.3(2), B(1); 332.8(2)°, B(2)) and 4 (342.2(2) Å) may be
54.94°. Of these, 5306 (Rint
= 0.0413) were considered observed (I
> 2.0s(I)) and were used to solve (direct methods) and refine (full matrix,
least squares on F2) the structure of 4. CCDC reference numbers 168666
and 168667. See http://www.rsc.org/suppdata/cc/b1/b106599h/ for crys-
tallographic data in CIF or other electronic format.
5
compared with the 339.8(2)° reported previously12 for (h -
5
C5Me5)Al?B(C6F5)3, suggesting that (h -C5Me5)Ga is a
5
slightly weaker Lewis base than (h -C5Me5)Al whereas (Dipp2-
nacnac)Ga appears to be a slightly stronger base than either
molecule. It remains to be seen if this is an accurate measure of
the Lewis basicity of (Dipp2nacnac)Ga where high steric effects
may also play a role.§
§ Note added in proof: after this work had been submitted, another paper
that described the synthesis and structure of 4 was published.13
1 (a) W. Uhl, W. Hiller, M. Layh and W. Schwartz, Angew. Chem., Int.
Ed. Engl., 1992, 31, 1364; (b) G. Linti and and W. Köster, J.
Organomet. Chem., 1996, 520, 107; (c) G. Linti and and W. Köster,
Angew. Chem., Int. Ed. Engl., 1997, 36, 2644; (d) W. Uhl and A.
Jantschak, J. Organomet. Chem., 1998, 555, 263; (e) G. Linti and A.
Rodig, Chem. Commun., 2000, 127.
2 D. Loos, E. Baum, A. Ecker, H. Schnöckel and A. J. Downs, Angew.
Chem., Int. Ed. Engl., 1997, 36, 860.
3 M. C. Kuchta, J. B. Bonnano and G. Parkin, J. Am. Chem. Soc., 1996,
118, 10914.
4 N. J. Hardman, B. Eichler and P. P. Power, Chem. Commun., 2000,
1991.
5 A. Haaland, M. Kjell-Gunnar, H. V. Volden, D. Loos and H. Schnöckel,
Acta. Chem. Scand., 1994, 48, 172.
6 N. J. Hardman, A. D. Phillips and P. P. Power, unpublished work.
7 C. L. B. Macdonald and A. H. Cowley, J. Am. Chem. Soc., 1999, 121,
12113.
8 P. Jutzi and G. Reumann, J. Chem. Soc., Dalton Trans., 2000, 2237 and
references therein.
9 H. Jacobsen, H. Berke, S. Döring, G. Kehr, G. Erker, R. Fröhlich and O.
Meyer, Organometallics, 1999, 18, 1724.
10 A. Haaland, M. Kjell-Gunnar, S. A. Shlykov, H. V. Volden, C.
Dohmeier and H. Schnöckel, Organometallics, 1995, 14, 3116.
11 (a) N. S. Hosmane, K.-J. Lu, H. Zhang and J. A. Maguire,
Organometallics, 1997, 16, 5163; (b) D. M. Schubert, M. A. Bandman,
W. S. Rees, C. B. Knobler, P. Lu, W. Nam and M. F. Hawthorne,
Organometallics, 1990, 9, 2046.
12 J. D. Gorden, A. Voigt, C. L. B. Macdonald, J. S. Silverman and A. H.
Cowley, J. Am. Chem. Soc., 2000, 122, 950.
13 P. Jutzi, B. Neumann, G. Reumann, L. O. Schebaum and H.-G.
Stammler, Organometallics, 2001, 20, 2854.
We are grateful to the National Science Foundation and the
Robert A. Welch Foundation for financial support and the
Albemarle Corporation for a generous gift of B(C6F5)3.
Notes and references
† All manipulations were carried out under anaerobic and anhydrous
conditions. 3: With rapid stirring, a pale yellow toluene solution (20 mL) of
(Dipp2nacnac)Ga (0.768 g, 1.5 mmol) was added dropwise to B(C6F5)3
(0.730 g, 1.5 mmol) in toluene (10 mL). After several minutes the solution
became colourless. The toluene was removed under reduced pressure and
the residue was dissolved in hexane (30 mL). The hexane solution was
concentrated to a volume of approximately 10 mL, and allowed to cool to
ca. 220 °C overnight. After 20 h, large colorless crystals of 3 were obtained
(1.21 g, 81%). mp 160–162 °C. 1H NMR (300 MHz, 298 K, C6D6) d 7.02
(t, p-H, 3JHH 7.5 Hz, 2H), 6.79 (d, m-H on phenyl, 3JHH 7.8 Hz), 4.99 (s, 1H,
methine CH), 2.81 (sept, 3JHH 6.6 Hz, 4H, CHMe), 1.39 (s, 6H, CMe), 1.02
3
3
(d, JHH 6.6 Hz, 12H, CHMe2), 0.88 (d, JHH 6.6 Hz, 12H, CHMe2): 11B
NMR (128 MHz, C6D6) d 220.30: 13C{1H} NMR (75 MHz, C6D6) d
170.72 (CN), 149.98 (br, o-C6F5), 146.82 (br, p-C6F5), 142.38 (CMe),
141.31 (o-C on phenyl; CCHCMe2), 138.56 (br, m-C6F5), 135.40 (br, ipso-
C6F5), 128.47 (p-C on phenyl), 124.63 (m-C on phenyl), 101.25 (g-C),
29.80 (CHMe2), 24.78 (CHMe2), 24.45 (CMe), 23.05 (CHMe2). 19F{1H}
NMR (376.0 MHz, 300 K, C6D6) d 2129.43 (m-C6F5), 2156.19 (p-C6F5),
2160.20 (o-C6F5). 4: A solution of B(C6F5)3 (0.62 g, 1.22 mmol) in 30 mL
of toluene was added to a pale yellow solution of [(C5Me5)Ga]6 (0.25 g, 1.22
mmol of (C5Me5)Ga units) in 20 mL of toluene at 278 °C. The stirred
yellow-coloured reaction mixture was maintained at 278 °C for 1 h,
following which it was allowed to warm slowly to room temperature and
stirred for an additional 4 h. The resulting tan coloured solution was filtered
Chem. Commun., 2001, 1866–1867
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