Reversible η6-η4 coordination in the association - Dissociation reactions of metallatricarbadecaboranyl complexes: An analogue of the η5-η3 cyclopentadienyl ring-slippage process [15]

Full text of DOI:10.1021/ja0022876

J. Am. Chem. Soc. 2000, 122, 11033-11034
11033
Reversible η6-η4 Coordination in the
Association-Dissociation Reactions of
Metallatricarbadecaboranyl Complexes: An
Analogue of the η5-η3 Cyclopentadienyl
Ring-Slippage Process
Bhaskar M. Ramachandran, Patrick J. Carroll, and
Larry G. Sneddon*
Department of Chemistry, UniVersity of PennsylVania
Philadelphia, PennsylVania 19104-6323
Figure 1. Comparison of structures and bonding modes of the tricar-
badecaborane and cyclopentadienide monoanions.
Fe(η6-[2-Ph-2,3,4-C3B7H9]) (3, blue) resulted in an immediate
color change characteristic of the formation of the nido-8-(η5-
C5Me5)Ru(CNBut)(η4-[9-Me-7,9,10-C3B7H9]) (4, yellow) and
nido-8-(η5-C5H5)Fe(CNBut)(η4-[9-Ph-7,9,10-C3B7H9]) (5, brown-
ish-red) products, respectively. Crystallization from the reaction
solutions gave 97% and 62% isolated yields, respectively, of pure
materials. Elemental analyses are consistent with the indicated
compositions resulting from the association of 1 equiv of the tert-
butyl isocyanide.
ReceiVed June 26, 2000
-
We have previously shown that the 6-R-nido-5,6,9-C3B7H9
anion is a versatile monoanionic ligand similar to the cyclopen-
tadienide anion, which can function as either an η6, 6-electron or
an η4, 4-electron donor to transition metals (Figure 1).1 We report
here synthetic and crystallographic studies of the first reversible
cage slippage between the η6-η4 coordination modes of the
tricarbadecaboranyl group that occurs upon the association-
dissociation reactions of ferra- and ruthenatricarbadecaboranyl
complexes with tert-butyl isocyanide. This reversible η6-η4
coordination is analogous to the η5-η3 ring slippage process that
is proposed to occur in related reactions of cyclopentadienyl metal
complexes.2
closo-1-(η5-C5Me5)Ru(η6-[2-Me-2,3,4-C3B7H9]) (1) + CNBut h
nido-8-[(η5-C5Me5)Ru(CNBut)](η4-[9-Me-7,9,10-C3B7H9]) (4)
(1)
The syntheses of ruthenatricarbadecaboranyl analogues of
ruthenocene, closo-1-(η5-C5Me5)Ru(η6-[2-R-2,3,4-C3B7H9]) [R )
Me (1), Ph (2)], were accomplished in 75% and 66% isolated
yields by the reaction of 6-R-nido-5,6,9-C3B7H9- with [Cp*RuCl2]x.
The synthesis of the isoelectronic ferrocene analogue, closo-1-
(η5-C5H5)Fe(η6-[2-Ph-2,3,4-C3B7H9]) (3) was achieved in 49%
yield by the reaction of the anion with (η5-C5H5)Fe(CO)2I.
Crystallographic determinations of 23 and 3 established the
structures shown in the ORTEP diagrams in Figure 2. As in
ruthenocene and ferrocene, the metals in 2 and 3 are sandwiched
between two 6-electron donating monoanionic ligands with the
metals having a formal +2 oxidation state. In both cases, the
metals show η6-coordination to the tricarbadecaboranyl cage and
are approximately centered over the puckered six-membered open
face. The closest metal cage interactions are with the two carbons,
C2 and C3, that are puckered out of the ring. Longer and
approximately equivalent bond lengths are observed between the
metals and the remaining four atoms (C4, B5, B6, and B7) on
the tricarbadecaboranyl bonding face.
closo-1-(η5-C5H5)Fe(η6-[2-Ph-2,3,4-C3B7H9]) (3) + CNBut h
nido-8-[(η5-C5H5)Fe(CNBut)](η4-[9-Ph-7,9,10-C3B7H9]) (5) (2)
Reactions 1 and 2 are reversible. Heating a toluene solution of
4 at reflux under flowing N2 for 20 min resulted in the quantitative
formation of 1, while simply dissolving pure 5 in glyme resulted
in the formation of an equilibrium mixture of 3 and 5. Heating
the solution to reflux under flowing N2 resulted in complete
conversion to 3.
The metals in 1, 2, and 3, as in ruthenocene and ferrocene,
have formal 18-electron counts. Thus, unless there is a change in
the donor properties of either the cyclopentadienyl or tricarba-
decaboranyl ligands, the metal-coordination of a 2-electron
isocyanide ligand would result in the formation of a 20-electron
complex. As shown in the ORTEP drawings in Figures 3 and 4,
crystallographic studies of 4 and 5 demonstrated that upon
isocyanide addition, the cyclopentadienyl rings remain sym-
metrically bonded to the metals with only a slight increase in the
metal to ring-centroid distances in 4 (1.871 Å) and 5 (1.736 Å)
relative to their values in 2 (1.829 Å) and 3 (1.695 Å),
respectively. However, the coordination mode of the tricarbade-
caboranyl ligand in 4 and 5 has changed from η6 to η4. Thus, in
4 and 5, the ruthenium and iron atoms are no longer centered
over the six-membered face, but instead, the metals have slipped
to one side of the cage. The metals are now centered above the
C7-B3-B4-C9 face with the bond lengths between the metal
and the four facial atoms being similar. Nonbonding distances
(>3.0 Å) are found between the metals and the C10 and B11
cage atoms. In 2 and 3, the M-C2-B8-B9-C3 atoms were
coplanar, but in 4 and 5 the metals lie significantly out of the
plane of the other four atoms, so that the dihedral angles between
the C7-M-C9 and C7-B2-B5-C9 planes in 4 and 5 are 36.8-
(1)° and 36.3(3)°, respectively.
As shown in eqs 1 and 2, addition of excess tert-butyl iso-
cyanide to glyme solutions of either closo-1-(η5-C5Me5)Ru(η6-
[2-Me-2,3,4-C3B7H9]) (1) (reddish-orange) or closo-1-(η5-C5H5)-
(1) (a) Plumb, C. A.; Carroll, P. J.; Sneddon, L. G. Organometallics 1992,
11, 1665-1671. (b) Plumb, C. A.; Carroll, P. J.; Sneddon, L. G. Organome-
tallics 1992, 11, 1672-1680. (c) Weinmann, W.; Wolf, A.; Pritzkow, H.;
Siebert, W.; Barnum, B. A.; Carroll, P. J.; Sneddon, L. G. Organometallics
1995, 14, 1911-1919. (d) Barnum, B. A.; Carroll, P. J.; Sneddon, L. G.
Organometallics 1996, 15, 645-654. (e) Barnum, B. A.; Carroll, P. J.;
Sneddon, L. G. Inorg. Chem. 1997, 36, 1327-1337. (f) Wasczcak, M. D.;
Hall, I. H.; Carroll, P. J.; Sneddon, L. G. Angew. Chem. Int. Ed. Engl. 1997,
36, 2227-2228.
(2) For some examples see: (a) Basolo, F. New J. Chem. 1994, 18, 19-
24 and references therein. (b) Basolo, F. Polyhedron 1990, 9, 1503-1535
and references therein. (c) O’Connor, J. M.; Casey, C. P. Chem. ReV. 1987,
87, 307-318 and references therein. (d) Schuster-Woldan, H. G.; Basolo, F.
J. Am. Chem. Soc. 1966, 88, 1657-1663. (e) Rerek, M. E.; Basalo, F. J. Am.
Chem. Soc. 1984, 106, 5908-5912. (f) Simanko, W.; Tesch, W.; Sapunov,
V. N.; Mereiter, K.; Schmid, R.; Kirchner, K; Coddington, J.; Wherland, S.
Organometallics 1998, 17, 5674-5688. (g) Calhorda, M. J.; Gamelas, C. A.;
Roma˜o, C. C.; Veiros, L. F. Eur. J. Inorg. Chem. 2000, 331-340.
(3) Only one of the two independent molecules found in the asymmetric
unit of the structural determination of 2 is shown. There were no significant
differences in the values of the bond distances and angles between the two
structures.
As we have previously discussed,1e,4 the change from the η6-
to η4-coordination mode corresponds to a conversion of the
tricarbadecaboranyl ligand from a 6- to a 4-electron donor. An
(4) Weinmann, W.; Pritzkow, H.; Siebert, W.; Sneddon, L. G. Chem. Ber.
Recl. 1997, 130, 329-333.
10.1021/ja0022876 CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000

11034 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Communications to the Editor
Figure 4. ORTEP drawing of the structure of nido-8-(η5-C5H5)Fe-
(CNBut)(η4-[9-Ph-7,9,10-C3B7H9]) (5). Selected bond lengths (Å):
Fe-C7 2.114(8), Fe-C9 2.102(8), Fe-B3 2.191(9), Fe-B4 2.175(9),
Fe-C12 1.854(8), C12-N13 1.170(10).
substitution reactions of numerous cyclopentadienyl complexes2
and some dicarbaboranyl complexes.7
Figure 2. ORTEP drawings of the structures of (a) closo-1-(η5-C5Me5)-
Ru(η6-[2-Ph-2,3,4-C3B7H9]) (2) and (b) closo-1-(η5-C5H5)Fe(η6-[2-Ph-
2,3,4-C3B7H9]) (3). Selected bond lengths (Å): 2, Ru-C2 2.111(3),
Ru-C3 2.073(4), Ru-C4 2.355(4), Ru-B7 2.358(4), Ru-B5 2.339(5),
Ru-B6 2.352(5); 3, Fe-C2 1.982(3), Fe-C3 1.955(3), Fe-C4 2.258-
(3), Fe-B7 2.275(3), Fe-B5 2.239(3), Fe-B6 2.253(3).
As discussed above for the tricarbadecaboranyl complexes,
cyclopentadienyl η5 to η3 ring slippage both provides an open
coordination site for the incoming ligand and avoids the formation
of an unfavorable 20-electron intermediate, thus providing a low-
energy pathway for the substitution reaction. Similar cyclopen-
tadienyl ring-slippages are thought to be critical steps in many
other important stoichiometric and catalytic organometallic reac-
tions.2 The η6 to η4 cage-slippage reaction reported herein not
only further illustrates the similarities of the cyclopentadienyl and
tricarbadecaboranyl ligands, but also demonstrates that tricarba-
decaboranyl cage-slippage is preferred over η5 to η3 cyclopen-
tadienyl ring-slippage in these complexes. The facile nature of
the tricarbadecaboranyl rearrangement further suggests that met-
allatricarbadecaboranyl complexes may exhibit even greater
reactivities than their cyclopentadienyl counterparts for many
transformations requiring the generation of a coordinatively
unsaturated metal center. Such reactivity studies are in progress
and will be reported in future publications.
Figure 3. ORTEP drawing of the structure of nido-8-(η5-C5Me5)Ru-
(CNBut)(η4-[9-Me-7,9,10-C3B7H9]) (4). Selected bond lengths (Å):
Ru-C7 2.218(5), Ru-C9 2.204(6), Ru-B3 2.273(6), Ru-B4 2.246(5),
Ru-C12 1.954(6), C12-N13 1.163(7).
Acknowledgment. We thank the National Science Foundation for
support of this research.
Supporting Information Available: Descriptions of the synthetic
methods and characterization data for compounds 1-5 and tables listing
refined positional and thermal parameters, bond distances, and bond angles
for compounds 2, 3, and 4 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
η4-tricarbadecaboranyl ligand could therefore be considered the
electronic analogue of a metal-coordinated η3-C3H5 allyl or a
1-
1-
“slipped” η3-C5H5 ligand. Thus, the net result of the addition
of the 2-electron isocyanide ligand to complexes 1 and 3 is to
convert the tricarbadecaboranyl ligands to their η4-coordination
modes reducing their electron donation to the metals to only 4
electrons and thereby preserving the metal’s favorable 18-electron
count.5 From a skeletal-electron counting point of view (Wade’s
rules6), this process simply corresponds to a cage-opening of the
metallatricarbadecaboranyl fragment brought about by the addition
of 2 electrons to the closo-M(R)C3B7H9 cage (24 skeletal
electrons) frameworks of 1 and 3 to form the nido-M(R)C3B7H9
cage (26 skeletal electrons) structures of 4 and 5.
JA0022876
(5) Siebert has also shown that the diborolyl rings in the formal 16-electron
complexes, (η5-C5Me5)Ru(R3′C3B2R2) and (η5-C5H5)Fe(R3′C3B2R2), reversibly
change their degree of folding upon the association/dissociation reactions of
the complexes with tert-butyl isocyanide to form the 18-electron (η5-C5Me5)-
Ru(CNBut)(R3′C3B2R2) and (η5-C5H5)Fe(CNBut)(R3′C3B2R2) complexes.
See: (a) Hettrich, R.; Kaschke, M.; Wadepohl, H.; Weinmann, W.; Stephan,
M.; Pritzkow, H.; Siebert, W.; Hyla-Kryspin, I.; Gleiter, R. Chem. Eur. J.
1996, 2, 487-494. (b) Mu¨ller, T.; Kaschke, M.; Strauch, M.; Ginsberg, A.;
Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 1999, 1685-1692.
(6) (a) Wade, K. AdV. Inorg. Chem. Radiochem. 1976, 18, 1-66. (b)
Williams, R. E. Chem. ReV. 1992, 92, 177-207 and references therein.
(7) See, for example: Shen, J. K.; Zhang, S.; Basolo, F.; Johnson, S. E.;
Hawthorne, M. F. Inorg. Chim. Acta 1995, 235, 89-97.
Although not observed for ferrocene and ruthenocene, revers-
ible ring slippage from η5 to η3 has been proposed, as shown in
the well-studied example2d,e below, to be a key step in the