[{(Fe(PEt3)3}2(μ-H)6B] [BPh4]: A complex containing octahedral hypercoordinate boron

Article abstract of DOI:10.1021/ic010477r

The isolation and crystallographic characterization of a dinuclear iron hydride containing a pseudo-octahedrally coordinated boron atom (1) is reported. A description of the bonding in the cation has been obtained from density functional calculations and a topological analysis.

Full text of DOI:10.1021/ic010477r

6334
Inorg. Chem. 2001, 40, 6334-6337
bonds.10 Owing to the unexpected NMR spectral data, a crystal
structure determination of 1 was undertaken.
[{Fe(PEt3)3}2(µ-H)6B][BPh4]: A Complex
Containing Octahedral Hypercoordinate Boron
Orange prisms of 1 were grown by slow diffusion of pentane
into a concentrated THF solution of 1 at -40 °C. Crystal-
lographic data are given in Table 1. The study revealed a discrete
dinuclear [{Fe(PEt3)3}2(µ-H)6B]+ cation with a separated [BPh4]-
counterion. The structure of the cation is shown in Figure 1
with the atomic numbering scheme and selected bond distances.
All the Fe-H bond distances are essentially the same, as are
the Fe-P distances, giving distorted octahedral environments
at each Fe atom. The P-Fe-Fe-P dihedral angles lie between
40° and 80°, i.e., the FeP3H3 units are effectively staggered,
but twisted 20° from the ideal geometry. The B(1)-H distances
are comparable, suggesting an octahedral environment for the
boron atom, but longer than expected from known structural
data on bridging hydrogens in borohydride complexes (generally
100-140 pm). The Fe-B distances are close to the sum of the
covalent radii of Fe and B (205 pm),11 corroborating the Fe-
B-Fe bonding interaction suggested by the 11B NMR spectrum.
The structure of 1 differs, therefore, from that reported for 2,
where notably the Ru-P(trans-H) bond is longer than those
Anna C. Hillier, Heiko Jacobsen, Dimitri Gusev,
Helmut W. Schmalle, and Heinz Berke*
Anorganisch-Chemisches Institut, Universita¨t Zu¨rich,-
Winterthurerstrasse 190, CH-8057 Zu¨rich, Switzerland
ReceiVed May 7, 2001
Introduction
A hypervalent1 molecule is described as one in which the
octet rule is not obeyed in the sense that there are more than
four pairs of electrons in the conventional Lewis diagram for
the molecule.2 The bonding in such molecules involves formal
expansion of the valence shell. Hypervalent compounds of the
first-row elements boron,3 carbon,4 and fluorine5 have been
reported. During our work on metal hydrides, we have isolated
an unusual iron hydride complex which appears to fall into this
category.6
-
trans to the bridging BH4 hydrogens and the P-Ru-Ru-P
dihedral angles vary from 1° to 110°. Short Ru-B distances
were, however, also reported. Our X-ray study suggests,
therefore, that the best description of the cation in 1 is [{Fe-
(PEt3)3}2(µ-H)6B]+.
Results and Discussion
The reaction of FeCl2 with PEt3 and NaBH4 in ethanol affords
dinuclear “[{Fe(PEt3)3}2(BH6)]+”, isolated as its BPh4 salt (1).
Venanzi has previously reported the related [{RuH(CH3C(CH2-
PPh2)3)}2(BH4)]+ (2).7 The temperature-invariant (183-323 K)
1H and 31P NMR spectra of 1 display only one hydride and one
phosphine environment. The broad hydride signal for 1 is at
rather high field for a B-H-M bridging hydrogen (δ -14.25
ppm), compared to δ -4.90 ppm for 2, although similar shifts
have been noted elsewhere.8 On decoupling from 31P, the
hydride signal sharpens, surprisingly since coupling between
bridging hydrogens and phosphine 31P nuclei is rarely observed
in transition metal borohydride complexes. No such sharpening
is observed in the 1H{11B} NMR spectrum. The broad Fe-B-
Fe signal at δ 60 ppm (ν1/2 ) 800 Hz) in the 11B NMR spectrum
is at the low-field end of the tridentate BX3 range,9 close to
shifts observed for boron atoms with direct metal-boron
In order to assist in the characterization of the unusual
coordination geometry of 1, we carried out density functional
calculations12 on the two model compounds [{Fe(PH3)3-
(H)3}2B]+ I and [{FeH(PH3)3}2(BH4)]+ II. Although modeling
real phosphine ligands by the prototypical PH3 molecule might
sometimes have implications on the relative energies of possible
isomers,13 molecular geometries are generally well represented
by the model systems, and one should also be able to establish
the characteristic bonding features around the boron center.
Optimized structures for the model compounds are displayed
in Figure 2. Both of these structures represent local minima on
the potential hypersurface of virtually the same energy, and our
calculations therefore suggest that both coordination geometries
might be viable alternatives for compound 1 and related species.
In I, we have six identical hydrogen ligands with almost equal
Fe-H and B-H bond distances around 155 pm, indicating D3d
local symmetry at boron, whereas in II, we find two single
Fe-H bonds, and four bridging hydrogens with distinctly
different d(Fe-H) and d(B-H) separations. The latter, at 136
(1) (a) Musher, J. I. Angew. Chem., Int. Ed. Engl. 1969, 8, 54. (b) Akiba,
K.-Y. Chemistry of HyperValent Compounds; Wiley-VCH: New York,
1999.
(2) Gillespie, R. J.; Robinson, E. A. Inorg. Chem. 1995, 34, 978 and
references therein.
(3) Lee, D. Y.; Martin, J. C. J. Am. Chem. Soc. 1984, 106, 5745.
(4) (a) Akiba, K.-y.; Yamashita, M.; Yamamoto, Y.; Nagase, S. J. Am.
Chem. Soc. 1999, 121, 10644. (b) G. Plah, A.; White, A. M.; O’Brien,
D. H. Chem. ReV. 1979, 70, 561. (c) Olah, G. A.; Rasul, G. Acc. Chem.
Res. 1997, 30, 245. (d) Schleyer, P. v. R.; Wu¨rthwein, E.-U.;
Kaufmann, E.; Clark, T.; Pople, J. A. J. Am. Chem. Soc. 1983, 105,
5930. (e) Kudo, H. Nature 1992, 355, 432. (f) Scherbaum, F.;
Grohmann, A.; Mu¨ller, G.; Schmidbaur, H. Angew. Chem., Int. Ed.
Engl. 1989, 28, 463.
(5) Ault, B. S.; Andrews, L. Inorg. Chem. 1977, 16, 2024.
(6) Gusev, D. G.; Hu¨bener, R.; Burger, P.; Orama, O.; Berke, H. J. Am.
Chem. Soc. 1997, 119, 3716.
(7) Rhodes, L. F.; Venanzi, L. M.; Sorato, C.; Albinati, A. Inorg. Chem.
1986, 25, 3337.
(8) (a) Ghilardi, C. A.; Innocenti, P.; Midollini, S.; Orlandini, A. J. Chem.
Soc., Dalton Trans. 1985, 605. (b) Jacobsen, G. B.; Andersen, E. L.;
Housecroft, C. E.; Hong, F.-E.; Buhl, M. L.; Long, G. J.; Fehlner, T.
P. Inorg. Chem. 1987, 26, 4040.
(9) Kennedy, J. D. In Multinuclear NMR; Mason, J., Ed.; Plenum Press:
New York, 1987.
(10) (a) Rath, N. P.; Fehlner, T. P. J. Am. Chem. Soc. 1988, 110, 5345, (b)
Fehlner, T. P.; Czech, P. T.; Fenske, R. F. Inorg. Chem. 1990, 29,
3103.
(11) Emsley, J. The Elements, 2nd ed.; Oxford University Press: Oxford,
1991.
(12) The BP86 calculations (Becke, A. D. Phys. ReV. 1988, A38, 3098;
Perdew, J. P. Phys. ReV. 1986, B33, 8822) utilized the program system
TURBOMOLE (Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel,
C. Chem. Phys. Lett. 1989, 162, 165; Treutler, O.; Ahlrichs, R. J.
Chem. Phys. 1995 102, 346; Eichkorn, K.; Treutler, O.; O¨ hm, H.;
Ha¨ser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 242, 652; Eichkorn,
K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997,
97, 119). A triple-ú valence basis plus polarization (TZVP) was
employed (Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994,
100, 5829), except for the H atoms of the phosphine groups, which
were treated with a split valence plus polarization basis set (SVP)
(Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571).
Optimized geometries and calculated frequencies are to be found in
the Supporting Information section.
(13) Jacobsen, H.; Berke H. Chem. Eur. J. 1997, 3, 881.
10.1021/ic010477r CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/20/2001

Notes
Inorganic Chemistry, Vol. 40, No. 24, 2001 6335
Table 1. Experimental Data for the X-ray Study of Compound 1
empirical formula
fw
temp, K
wavelength, Å
space group
unit cell dimens
a, Å
C60H116B2P6Fe2
1156.67
183(2)
0.71073
P1h
11.7396(8)
15.9566(11)
17.8324(12)
77.947(8)
83.942(8)
89.176(8)
3248.5(4)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
F
calcd, mg m-3
1.183
6.29
µ(Mo KR), cm-1
final R indices [I > 2σ(I)]
R1 ) 0.0472, wR2 ) 0.1026
Figure 2. Optimized BP86 geometries and selected bond distances
(in pm) for the [{Fe(PH3)(H)3}2B]+ model compounds I (above) and
II (below) (P ) PH3).
becomes more and more clear that this is an unnecessary
concept,2 which mystifies rather than clarifies the bonding
situation. Thus, it would be better to talk about hypercoordi-
nation instead, in particular when considering that an orbital
analysis of so-called hypervalent compounds leads to the
conclusion that the octet rule per se is not violated.17 For
structure I, a partial orbital diagram is presented in Figure 3,
which also contains sketches18 of those molecular orbitals
responsible for Fe-H and B-H bonding. The degenerate 1eu
set is made up of Fe-H bonding orbitals on each of the metal
centers, which interact with boron-based px and py orbitals,
respectively. The nature of the metal fragment orbitals can
clearly be seen in the corresponding 1eg orbital, which for
symmetry reasons does not bear any boron contributions. The
combination 2eu, which is B-H bonding, but Fe-H antibond-
ing, constitutes the LUMO of I, which underlines the fact that
the occupied orbital 1eu to a major extent contributes to Fe-H
Figure 1. ORTEP view of cation 1. Carbon and non-hydride hydrogen
atoms are omitted for clarity. Relevant bond lengths (in pm): Fe(1)-
B(1) 190.3(3); Fe(2)-B(1) 191.3(3); Fe(1)-P(1) 223.67(8); Fe(1)-
P(2) 221.92(6); Fe(1)-P(3) 222.64(6); Fe(2)-P(4) 223.28(7); Fe(2)-
P(5) 224.62(7); Fe(2)-P(6) 223.74(8); Fe(1)-H(1) 154(2); Fe(1)-H(2)
154.7(18); Fe(1)-H(3) 150.0(19); Fe(2)-H(4) 152.3(19); Fe(2)-H(5)
153.0(19); Fe(2)-H(6) 154(2); B(1)-H(1) 148.2(19); B(1)-H(2)
155.8(19); B(1)-H(3) 153(2); B(1)-H(4) 149.2(19); B(1)-H(5) 159-
(2); B(1)-H(6) 150.1(19).
pm, are well within the usual range of B-H distances of µ-H
ligands in tetrahydroborate complexes. The Fe-B bond is
somewhat longer in II, and the Fe-P bond trans to the hydride
ligand is elongated by 4 pm, compared to the other phosphine
groups. These observations, together with the closer match of
the P-M-M-P torsion angles in 1 with those for I than for
II, indicate a pseudo-octahedral arrangement of hydrogens
around boron in 1. A recent theoretical study suggests that the
2
bonding interaction. The orbital 1a2u is dominated by dz and pz
functions at iron, as well as pz at boron, and thus constitutes
Fe-B bonding. The next orbital, 2a2u, which does not carry
any contribution from boron, again is Fe-H bonding in
character. From this orbital analysis one might draw the
conclusion that system I should be described as two anionic
[(PH3)3FeH3]- units linked by a B3+ center, and that the Fe-B
bond is supported by secondary B-H interactions. A topological
analysis19 of the electron density F(r) supports this description.
In particular, gradient paths of the gradient vector field ∇F(r),
which originate at a bond critical point and terminate at the
nuclei, might be used to define a chemical bond, and to establish
a molecular graph. Such a graph for the bridging region of I is
+
BH6 cation should be viewed as containing two dihydrogen
units, i.e., as [B(H)2(H2)2]+ rather than as an octahedral unit.14
The difference in our case presumably arises from the presence
of the Fe(PEt3)3 moieties and the strong Fe-H interaction.
The question remains as to how the chemical bonding around
the boron center might be characterized. In II, the main
structural motif is a [BH4]- anion bridging two metal moieties.
The structure and bonding of transition metal tetrahydroborato
complexes has recently been reviewed,15 and we refer the reader
to the literature for a detailed description of the orbital
interactions of a [BH4]- unit bridging two metal centers. Of
more interest is the D3d geometry I, which appears to be an
example for a hypervalent boron center. However, a caveat is
in order. While the term hyperValence still is one of the main
bonding concepts taught in inorganic chemistry curricula,16 it
(16) See, for example: Shriver, D. F.; Atkins, P. W.; Langford, C. H.
Inorganic Chemsitry, 2nd ed.; Oxford University Press: Oxford, 1994.
(17) Albright, T. A.; Burdett, J. K.; Whangboo, M.-H. Orbital Interactions
in Chemistry; John Wiley: New York, 1985; Chapter 14.
(18) The orbital pictures were generated with the help of the program
Molden (Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des.
2000, 14, 123.)
(19) (a) Bader, R. W. F. Atoms in molecules. A quantum theory; Clarendon
Press: Oxford, 1990. (b) Popelier, P. L. A. Atoms in molecules. An
introduction; Pearson Education: Harlow, 1999.
(14) Jursic, B. S. J. Mol. Struct. (THEOCHEM) 1999, 492, 97.
(15) Xu, Z. T.; Lin, Z. Y. Coord. Chem. ReV. 1996, 156, 139.

6336 Inorganic Chemistry, Vol. 40, No. 24, 2001
Notes
certain similarities between the agostic M-H bond as found
for transition metal complexes,23 and the B-H bond in I.
Conclusions
Our calculations point to the fact that the prevailing structural
motif in 1 is the pseudo-octahedral environment at the central
boron. This is in accord with the outcome of the X-ray structure
determination. The lowering of the symmetry from Oh to D3d
is presumably responsible for the broad 11B NMR signal. A
topological analysis has revealed the nature of the weak B-H
bonding interactions which stabilize the structure by establishing
a pseudo-octahedral surrounding of H ligands at boron. This
unusual hypercoordination of a boron atom by hydrides is
unprecedented, making 1 an unusual example of a boron
analogue of the so-called hypervalent second-period main group
species.
Experimental Section
General Procedures and Measurements. All operations were
carried out under nitrogen atmosphere using standard Schlenk line and
glovebox techniques. Solvents were distilled from Na/Ph2CO (THF,
pentane) (THF was predried over KOH). Absolute ethanol and methanol
over molecular sieves were purchased from Fluka and degassed by
bubbling dry dinitrogen through the solvent in a syringe immediately
prior to injection. C4D8O used in NMR experiments was predried over
sodium, then dried, and stored over sodium/potassium alloy (3:1 Na/
K). The following reagents were purchased from commercial suppliers
and used as received: FeCl2 (anhydrous), Na[BPh4] (Fluka), NaBH4
1
(Aldrich), PEt3 (Alfa). H, 13C, 31P, and 11B NMR experiments were
carried out on a Varian Gemini-300 (operating at 300.1, 75.4, 121.5,
and 96.2 MHz, respectively), or a Bruker DRX500 spectrometer
(operating at 500.2, 125.8, 202.5, and 160.5 MHz, respectively). H
Figure 3. Orbital energy levels for I, labeled according to D3d
symmetry. Selected orbitals are also depicted, for which the corre-
sponding energy lines appear in bold.
1
and 13C{1H} NMR spectra were referenced to the residual proton or
13C resonances of the deuterated solvent. 31P chemical shifts were
externally referenced to 85% H3PO4 and 11B chemical shifts to BF3/
OEt2. IR spectra were recorded on a BIO RAD FTS-45 spectropho-
tometer as KBr pellets. Raman measurements were made on a Renishaw
Ramascope instrument, on samples sealed under nitrogen in capillary
tubes.
Synthesis of [{Fe(PEt3)3}2(µ-H)6B][BPh4] (1). To a solution of
FeCl2 (0.40 g, 3.16 mmol) in ethanol (20 mL) was added PEt3 (1.3
mL, 9.47 mmol); then the mixture was stirred for 20 min and cooled
to -50 °C, and NaBH4 (0.18 g, 4.75 mmol) was added. The deep red
mixture was stirred for 6 h while warming to room temperature, during
which time it turned orange/brown. After filtration, solvent was removed
from the filtrate in vacuo and the yellow product washed with hexane
(3 × 20 mL) and then dried under dynamic vacuum. The product was
dissolved in methanol (15 mL), and then NaBPh4 (0.53 g, 1.58 mmol)
was added to form a yellow precipitate. After stirring for 15 min, the
precipitate was allowed to settle and the solvent decanted off. The
precipitate was washed with a further 2 × 20 mL of methanol, then
dried, dissolved in THF, filtered over Celite, and dried in vacuo to
afford 0.25 g (0.22 mmol, 14%) of 1. Elemental microanal. Calcd for
C60H116P6B2Fe2: C, 62.30; H, 10.11. Found: C, 62.32; H, 10.31. IR
(KBr pellet, cm-1): 1884 (s, br) νBH/νFeH. Raman (solid, cm-1): 1931
(m, br) νBH/νFeH. 1H NMR (300 MHz, [D8]THF, 293 K): δ 7.26 (br s,
8 H, BPh4); 6.83 (t, 8 H, BPh4); 6.68 (t, 4 H, BPh4); 1.67 (m, 36 H,
P(CH2CH3)3); 1.12 (m, 54 H, P(CH2CH3)3); -14.25 (br, 6 H, Fe-H).
31P NMR (121.47 MHz, [D8]THF, 293 K): δ 49.53 (s, PEt3). 13C NMR
(75.47 MHz, [D8]THF, 293 K): δ 137.56 (s, BPh4); 125.91 (s, BPh4);
122.01 (s, BPh4); 24.26 (m, PCH2CH3); 8.87 (s, PCH2CH3). 11B NMR
(160.49 MHz, [D8]THF, 293 K): δ 60 (br, Fe-B-Fe); -6.81 (s, BPh4).
X-ray Structure Analysis for 1. Crystal data and experimental
details are listed in Table 1. Orange crystals were obtained by slow
Figure 4. Contour lines of the charge density (thin) and the molecular
graph (bold) in the bridging region of I. Bond critical points are denoted
by squares (9).
shown20 in Figure 4. We see bond paths, which connect the
µ-H ligands with the iron as well as the boron centers. The
curvature of the B-H path indicates that this bond is different
from a typical covalent linkage. This conclusion is corroborated
by an inspection of the bond ellipticities ꢀ,21 which not only
characterize the extent to which charge is preferentially ac-
cumulated but also provide a measure for structural stability.22
The calculated ꢀ values for the Fe-B, Fe-H, and B-H bonds
amount to 0.01, 0.65, and 1.39 au and point to a structural
instability of the B-H bond. The topological analysis reveals
(20) The topological analysis was performed using the program MORPHY
(Popelier, P. L. A. Comput. Phys. Commun. 1996, 92, 212).
(21) Bond ellipticities ꢀ are defined by the two principal curvatures λ1 and
λ2 of F(r) at the bond critical point: ꢀ ) (λ2/λ1) - 1.
(22) Cremer, D.; Kraka, E.; Slee, T. S.; Bader, R. F. W.; Lau, C. D. H.;
Nguyen-Dang, T. T.; MacDougall, P. J. J. Am. Chem. Soc. 1983, 105,
5069.
(23) (a) Popelier, P. L. A.; Logothetis, G. J. Organomet. Chem. 1998, 555,
101. (b) Scherer, W.; Hieringer, W.; Spiegler, M.; Sirsch, P.; McGrady,
S.; Downs, A. J.; Haaland, A.; Pedersen, B. Chem. Commun. 1998,
2471.

Notes
Inorganic Chemistry, Vol. 40, No. 24, 2001 6337
diffusion of pentane into a THF solution of 1 at -40 °C. A crystal
(dimensions 0.47 × 0.37 × 0.33 mm) covered with hydrocarbon oil
was mounted on top of a glass fiber and immediately transferred to
the goniometer of a Stoe IPDS diffractometer, where it was cooled to
183(2) K using an Oxford Cryo System; 45324 reflection intensities
were measured with graphite-monochromated Mo KR radiation (λ )
0.71073 Å). φ-rotation scan, 1.1° rotations per image, total φ-rotation
set to 240°, resulted in 218 images. Data were corrected for Lorentz
and polarization effects and for absorption (numerical, 20 indexed
crystal faces, transmission 0.8193-0.7565). The structure was solved
with 17714 unique data (Rint ) 0.0451) with direct methods using
SHELXS-97.24 The structure was refined by full-matrix least-squares
methods on F2 (wR2 ) 0.1128) for all unique data and 655 parameters,
R1 ) 0.0472 for 10591 data with I g 2σ(I), R1 for 17714 data was
0.0870. Three disordered ethyl groups were refined with partitioned
positions with PART instruction of SHELXL-97. The six hydride atoms
were located by difference electron density calculations. Their positions
and isotropic displacement parameters were refined using soft restraint
distances which were obtained from DFT calculations. All other
positions of H atoms were calculated after each refinement cycle (riding
model). The final maximum and minimum residual electron densities
were 1.292 and -0.888 Å-3
.
Acknowledgment. This work was supported by the Uni-
versity of Zu¨rich and the Swiss National Science Foundation.
We thank Dr. Thomas Fox for the NMR decoupling experi-
ments. We also thank our referees for valuable suggestions.
Supporting Information Available: Optimized geometries and
vibrational spectral data for I and II. This material is available free of
charge via the Internet at http://pubs.acs.org. X-ray data: CCDC No.
151848.
(24) Sheldrick, G. M. SHELX-97: Software package for crystal structure
determination: Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997.
IC010477R