Two-coordinate first row transition metal complexes with short unsupported metal-metal bonds

Article abstract of DOI:10.1021/ja1089777

A series of first row transition metal complexes with unsupported M-Fe bonds, (3,5-ⁱPr₂-Ar*)MFe(η⁵-C ₅H₅)(CO)₂ (M = Fe (1), Mn (2), Cr (3), 3,5-ⁱPr₂-Ar* = -C₆H-2,6-(C ₆H₂-2,4,6-ⁱPr₃)₂-3,5- ⁱPr₂), was synthesized by salt metathesis. They were characterized by ¹H NMR, UV-vis spectroscopy, X-ray crystallography, and SQUID magnetic measurements. Two distinct Fe atoms in 1 were confirmed by Moessbauer spectroscopy. All three compounds feature short metal-metal bond distances (Fe-Fe, 2.3931(8) A (1); Mn-Fe, 2.4512(5) A (2); Cr-Fe, 2.4887(5) A (3)). Their DFT computed structures were in excellent agreement with the experimental data and revealed a dative bonding interaction between the metals.

Full text of DOI:10.1021/ja1089777

Published on Web 11/19/2010
Two-Coordinate First Row Transition Metal Complexes with Short
Unsupported Metal-Metal Bonds
Hao Lei,† Jing-Dong Guo,‡ James C. Fettinger,† Shigeru Nagase,‡ and Philip P. Power*,†
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616, United States,
and Department of Theoretical and Computational Molecular Science, Institute for Molecular Science,
Okazaki, Aichi 444-8585, Japan
Received October 5, 2010; E-mail: pppower@ucdavis.edu
Scheme 1. Synthesis of 1, 2, and 3
Abstract: A series of first row transition metal complexes with
unsupported M-Fe bonds, (3,5-iPr2-Ar*)MFe(η5-C5H5)(CO)2 (M
) Fe (1), Mn (2), Cr (3), 3,5-iPr2-Ar* ) -C6H-2,6-(C6H2-2,4,6-iPr3)2-
3,5-iPr2), was synthesized by salt metathesis. They were char-
1
acterized by H NMR, UV-vis spectroscopy, X-ray crystallogra-
phy, and SQUID magnetic measurements. Two distinct Fe atoms
in 1 were confirmed by Mo¨ssbauer spectroscopy. All three
compounds feature short metal-metal bond distances (Fe-Fe,
2.3931(8) Å (1); Mn-Fe, 2.4512(5) Å (2); Cr-Fe, 2.4887(5) Å
(3)). Their DFT computed structures were in excellent agreement
with the experimental data and revealed a dative bonding
interaction between the metals.
4.5 to 5.0 µB at 5-300 K (Figure S1), suggesting an S ) 2 ground
state with little first-order orbital angular momentum contribution.
Zero-field Mo¨ssbauer spectroscopy at 300 and 190 K (Figure S3
and Table S1) revealed the presence of two different Fe atoms. A
signal (δ ) 0.42 mm/s, ∆EQ ) 0.96 mm/s at 190 K) was consistent
with two-coordinate Fe(II).11 Another signal (δ ) 0.08 mm/s, ∆EQ
) 1.73 mm/s at 190 K) was consistent with an Fe atom in an [(η5-
C5H5)Fe(CO)2] fragment.12 In addition, a signal (δ ) 0.15 mm/s,
∆EQ ) 1.94 mm/s at 190 K) indicated the presence of some [(η5-
C5H5)Fe(CO)2]2 dimer (literature value: δ ) 0.227(2) mm/s, ∆EQ
) 1.912(5) mm/s at 77 K),12 whose presence could be due to
disproportionation or oxidation of 1.
X-ray crystallography confirmed the formulation of 1 which includes
an unsupported Fe-Fe bond (Figure 1). The Fe(1)-Fe(2) distance
(2.3931(8) Å) is much shorter than those in other known species with
unsupported Fe-Fe bonds: e.g., [{Fe(tim)}2] (2.6869(6) Å),6 [(η3-
C3H5)Fe(CO)3]2 (3.138(3) Å),5a {[(C5H5N)4Na]2[Fe2(CO)8]}∞ (2.815(1)
Å),5b [Fe(CO2tBu)(CO)4]2 (2.840(1) Å),5c and (N-methyllutidinium)2-
Understanding the nature of metal-metal bonds is of funda-
mental chemical importance.1 Among the numerous complexes
containing a direct metal-metal bond, those unsupported by
bridging ligands have attracted particular interest because of their
inherent simplicity.2 A number of first row transition metal species
with bridged homo- or heteronuclear metal-metal bonds are known,
but unbridged complexes are less numerous3,4 and are almost ex-
clusively stabilized by π-acid ligands at each metal.5 Recently, Wie-
ghardt and co-workers reported a dimeric compound [{Fe(tim)}2]
(tim ) 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-
tetraene), which contains an unbridged Fe-Fe bond.6 In addition,
the group of Roesky reported the synthesis of [{HC(CMeNAr)2}-
Mn]2 (Ar ) -C6H3-2,6-iPr2), which has an unsupported Mn-Mn
bond.7 These compounds were prepared by reduction of ligand-
substituted metal salts with alkali metals. Our attempts to prepare
an Fe-Fe bonded complex stabilized by the large terphenyl ligand
Ar′ (Ar′ ) -C6H3-2,6-(C6H3-2,6-iPr2)2) via reduction of [Ar′Fe(µ-
Br)]2 led to a low yield of the dimeric product [Ar′FeFeAr′],8 in
which the Fe-Fe unit was supported by a bridging terphenyl ligand
incorporating strong η6-metal-flanking-ring interactions to each iron.
We now show that an unsupported Fe-Fe bond can be constructed
by the reaction of [(3,5-iPr2-Ar*)Fe(µ-Cl)]2 (3,5-iPr2-Ar* ) -C6H-
2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2)9 with K[(η5-C5H5)Fe(CO)2].10 We
also show that the corresponding unbridged heterometallic Mn-Fe
and Cr-Fe species can be prepared in a similar fashion.
Treatment of [(3,5-iPr2-Ar*)Fe(µ-Cl)]2 with 2 equiv of K[(η5-
C5H5)Fe(CO)2] at ca. -78 °C gave, after workup and recrystalli-
zation from hexane, dark red crystals of (3,5-iPr2-Ar*)FeFe(η5-
1
C5H5)(CO)2 (1) in 64% yield (Scheme 1). The H NMR spectrum
Figure 1. Solid state molecular structure of 1 or 2 (M ) Fe (1) or Mn (2);
H atoms and solvent molecules are not shown; thermal ellipsoids are shown
at 30% probability). Selected bond distances (Å) and angles (deg) of 1
(corresponding data for 2 are shown in brackets): M(1)-Fe(2) 2.3931(8)
[2.4512(5)], M(1)-C(1) 2.022(4) [2.081(2)], C(1)-M(1)-Fe(2) 163.94(12)
[166.82(6)], M(1)-C(1)-C(2) 107.9(3) [109.33(15)], M(1)-C(1)-C(6)
133.6(3) [131.99(17)], C(2)-C(1)-C(6) 118.5(4) [118.7(2)].
of 1 exhibits paramagnetically shifted resonances which span the
chemical shift range +66 to -69 ppm. The µeff of 1 ranged from
† University of California.
‡ Institute for Molecular Science.
9
10.1021/ja1089777  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 17399–17401 17399

C O M M U N I C A T I O N S
[Fe2(CO)8] (2.780(1) Å).5d It is also shorter than the 2.52 Å interatom
distance in iron metal.13 When it is compared to the Fe-Fe lengths in
bridged compounds (2.2-3.12 Å),1,14 the Fe-Fe distance in 1 lies in
the shorter end of the range. It is noteworthy that the reported shortest
Fe-Fe bonds (2.198(2) Å, Fe2(DPhBz)3, HDPhBz ) N,N′-diphenyl-
benzamidine; 2.2318(8) Å, Fe2(DPhF)3, HDPhF ) N,N′-diphenylfor-
mamidine) were calculated to have a bond order of 1.5.14a The
coordination at Fe(1) deviates from linear geometry to afford a
C(1)-Fe(1)-Fe(2) angle of 163.94(12)°. This is similar to those
observed in a number of mononuclear, two-coordinate iron
species, such as Fe{N(SiMePh2)2}2 (169.0(1)°),15 Fe{N(C6H3-2,6-
iPr2)(CH2tBu)}2 (168.8(2)°),16 Fe(NMesBMes2)2 (166.6(1)°, Mes )
2,4,6-Me3C6H2),17 Fe(C6H3-2,6-Mes2)2 (164.44(12), 171.06(12)°),18
and FeAr′2 (159.34(6)°).19 The Fe(1)-C(1) distance (2.022(4) Å) in
1 is similar to the Fe-C σ-bond lengths in [Ar′FeFeAr′] (2.028(4)
and 2.048(4) Å),8 Fe(C6H3-2,6-Mes2)2 (2.040(3) Å),18 FeAr′2 (2.059(1)
Å),19 and (η6-C6H6)Fe(Ar*-3,5-iPr2) (2.029(4) Å).9 The angles at C(1)
are distorted from idealized trigonal values (e.g., Fe(1)-C(1)-C(2)
107.9(3)°, Fe(1)-C(1)-C(6) 133.6(3)°), possibly due to a secondary
interaction between Fe(1) and the ipso-carbon of a flanking ring
(Fe(1)-C(7) 2.686(4) Å).
Figure 2. Representation of solid state structure of 3 with selected bond
distances (Å) (H atoms and isopropyl groups are not shown for clarity;
thermal ellipsoids are shown at 30% probability). Selected bond angles (deg):
C(1)-Cr(1)-Fe(1) 141.31(6), Cr(1)-C(1)-C(2) 98.52(14), Cr(1)-C(1)-C(6)
143.22(16), C(2)-C(1)-C(6) 118.25(19).
To test the generality of the salt metathesis route, the reactions
between [(3,5-iPr2-Ar*)M(µ-Cl)]2 (M ) Mn,9 Cr20) and K[(η5-
C5H5)Fe(CO)2] were investigated. The reaction with [(3,5-iPr2-
Ar*)Mn(µ-Cl)]2 yielded yellow crystals of (3,5-iPr2-Ar*)MnFe(η5-
C5H5)(CO)2 (2) in 57% yield upon recrystallization from hexane
(Scheme 1). In addition, a small portion of [(η5-C5H5)Fe(CO)2]2
cocrystallized during the preparation (identity confirmed by X-ray
crystallography), suggesting a lower stability of 2 in comparison
to 1. Magnetic studies of 2 revealed a nearly ideal Curie
paramagnetism behavior with µeff ) 5.1 to 5.2 µB at 5-300 K
(Figure S2). This is consistent with high-spin Mn(II) but may be
lower than the idealized spin-only value 5.92 µB perhaps because
of the inclusion of the above-mentioned diamagnetic impurity [(η5-
C5H5)Fe(CO)2]2.
Figure 3. Graphic representation of the optimized structure of 1 (Fe, blue;
O, red; C, black; H, white). A comparison of selected calculated and
experimental bond distances and angles is given in Table 1.
The X-ray crystal structure of 2 showed that it is essentially
isomorphous and isostructural to 1 in the solid state (Figure 1). It
features an unsupported Mn-Fe bond. The only examples of unsup-
ported Mn-Fe bonds are found in mixed metal carbonyl derivatives21
or clusters.22 The Mn(1)-Fe(1) distance in 2 is 2.4512(5) Å, which is
shorter than the other known unsupported Mn-Fe bonds (2.601-2.843
Å).21,22 Similar to 1, the C(1)-Mn(1)-Fe(1) angle in 2 (166.82(6)°)
distorts from linearity and is comparable to the corresponding
angles in Mn(C6H3-2,6-Mes2)2 (166.41(13), 173.00(13)°),18 Mn-
(NMesBMes2)2 (160.4(2)°),23 and Mn{N(SiMePh2)2}2 (170.7(1)°).24
The Mn(1)-C(1) distance (2.081(2) Å) is comparable to those in (µ-
η6:η6-C7H8){Mn(Ar*-3,5-iPr2)}2(2.088(3)and2.089(3)Å)9andMn(C6H3-
2,6-Mes2)2 (2.095(3) Å).18 The secondary metal-carbon distance is
longer (Mn(1)-C(7) 2.788(3) Å) than that in 1, and there is a lower
degree of distortion from the trigonal planar geometry at the C(1) atom
(Mn(1)-C(1)-C(2) 109.33(15)°, Mn(1)-C(1)-C(6) 131.99(17)°).
Å),25a and (η5-C5H5)2CrFe(CO)5 (2.901(1) Å).25b In contrast to the
two-coordination of Fe(1) in 1 and of Mn(1) in 2, the Cr(1) atom
in 3 affords a quasi trigonal planar geometry (C(1)-Cr(1)-Fe(1)
141.31(6)°), due to the relatively close interaction between Cr(1)
and C(13) (Cr(1)-C(13) 2.365(2) Å). There is a large difference
between the two exocyclic bond angles at C(1) (Cr(1)-C(1)-C(2)
98.52(14)°, Cr(1)-C(1)-C(6) 143.22(16)°). The Cr(1)-C(1) dis-
tance (2.061(2) Å) is slightly shorter than those in the Cr(I) species
[(3,5-iPr2-Ar*)Cr(L)] (L ) THF, 2.087(3) Å; L ) PMe3, 2.116(2)
Å)20 and Ar′CrCrAr′ (2.131(1) Å),26 but is similar to that in the
Cr(II) derivative [Ar′Cr(µ-Cl)]2 (2.041(3) Å).27 The room temper-
ature µeff of 3 was determined to be 4.3 µB by the Evans′ method,
which is consistent with high-spin d4 Cr(II). The electronic spectrum
of 3 features an absorption at 504 nm (470 mol-1 L cm-1), which
is similar to that of 1 (504 nm, 460 mol-1 L cm-1). In contrast, 2
displays a weak shoulder absorption at 446 nm. In addition, all
three compounds display a much higher CO stretching frequency
(1958, 1902 (1); 1962, 1891 (2); 1933, 1879 (3) cm-1) in
comparison to the ionic salt [nBu4N][(η5-C5H5)Fe(CO)2] (1865, 1788
cm-1),28 suggesting a significant degree of electron donation from
the [(η5-C5H5)Fe(CO)2] to the [(3,5-iPr2-Ar*)M] fragment.
The corresponding Cr-Fe complex (3,5-iPr2-Ar*)CrFe(η5-
C5H5)(CO)2 (3) was isolated in an analogous manner to 1 and 2.
Red crystals of 3 were obtained by the reaction of [(3,5-iPr2-
Ar*)Cr(µ-Cl)]2 with K[(η5-C5H5)Fe(CO)2] followed by recrystal-
lization from hexane (Scheme 1). The solid state structure of 3
(Figure 2), determined by X-ray crystallography, features an
unsupported Cr-Fe bond. Binuclear compounds containing Cr-Fe
bonds are relatively scarce, and unsupported Cr-Fe bonded species
are limited to three carbonyl derivatives.25 The Cr(1)-Fe(1)
distance in 3 (2.4887(5) Å) is much shorter than the observed values
in these structures: [PPN]2[FeCr(CO)9] (2.941(2) Å, PPN )
bis(triphenylphosphine)iminium),25a [PPN][HFeCr(CO)9] (2.956(7)
To gain further insight into the electronic structures of 1 and 2,
DFT calculations were performed at the spin-unrestricted B3LYP
level. The optimized structures of 1 (Figure 3) and 2 (Figure S5)
are in excellent agreement with their crystallographic data. As
illustrated in Table 1, an Fe-Fe distance of 2.403 Å was calculated
for 1. The calculated Mn-Fe distance of 2.458 Å for 2 is also
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C O M M U N I C A T I O N S
Table 1. Calculated and Experimental Bond Distances (Å) and
Angles (deg) for 1
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Experimental
Calculated
Fe(1)-Fe(2)
2.3931(8)
2.022(4)
163.94(12)
107.9(3)
133.6(3)
118.5(4)
2.403
2.039
166.5
110.3
130.0
119.7
Fe(1)-C(1)
C(1)-Fe(1)-Fe(2)
Fe(1)-C(1)-C(2)
Fe(1)-C(1)-C(6)
C(2)-C(1)-C(6)
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close to the experimental value of 2.4512(5) Å (Table S4). The
natural charges for Fe(1) and Fe(2) in 1 are +1.12 and -1.35,
respectively, clearly indicating an intramolecular electron transfer
from Fe(1) to Fe(2). A similar effect was observed in 2, where
natural charges for Mn(1) and Fe(1) of +1.18 and -1.34,
respectively, were calculated. Calculated natural spin densities of
Fe(1) in 1 (3.79) and Mn(1) in 2 (4.87) are consistent with the
SQUID magnetic characterization and the +2 oxidation states for
the two unsaturated metal centers. The low Wiberg bond order for
1 (0.36) and 2 (0.34) indicates limited covalent interactions between
the metals. Investigations of the frontier orbitals (Figures S6 and
S7) suggested that a dative bond (HOMO) was formed after the
formal electron transfer (one electron from Fe(1) to Fe(2) in 1;
Mn(1) to Fe(1) in 2). In addition, the UV-vis absorption maxima
of 1 and 2 were predicted to be at 561 and 451 nm, respectively,
and each absorption is contributed from a mixture of several
transitions (Figures S8 and S9).
In summary, the synthesis of binuclear complexes containing
the shortest unsupported M-Fe (M ) Fe, Mn, Cr) bonds was
achieved by a salt metathesis route. Extension of this synthetic route
to other metal-metal combinations and investigation of the
reactivity of these M-Fe bonds toward small molecules are
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Acknowledgment. We are grateful to the National Science
Foundation (CHE-0948417) and the Specially Promoted Research
of MEXT (Japan) for financial support. We thank Dr. Peter Klavins
for help with SQUID measurements. We also thank Dr. Saeed
Kamali and Prof. Stephen P. Cramer for assistance with Mo¨ssbauer
spectroscopy characterization.
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Supporting Information Available: Experimental procedures,
computational details, Mo¨ssbauer spectra and fitting parameters of 1,
graphical representations of the magnetic data, and crystallographic data
of 1-3 (in CIF format). This material is available free of charge via
the Internet at http://pubs.acs.org.
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