Metal-metal multiple bonds in early/late heterobimetallics support unusual trigonal monopyramidal geometries at both Zr and Co

Article abstract of DOI:10.1021/ja909310t

(Chemical Equation Presented) Reduction of Zr/Co heterobimetallic complexes ICo(MesNPⁱPr₂)₃ZrCl (1) and ICo( ⁱPrNPⁱPr₂)₃ZrCl (2) with excess Na/Hg under N₂, followed by subsequent benzene extraction to remove coordinated Na halide salts, leads to neutral two-electron reduced, dinitrogen-bound complexes (THF)Zr(MesNPⁱPr₂) ₃Co-N₂ (4) and Zr(ⁱPrNPⁱPr ₂)₃Co-N₂ (5). Upon halide loss, a THF solvent molecule coordinates to the axial site of the Zr center in 4, while this axial site remains unoccupied in 5. X-ray crystallography reveals short Co-Zr distances in 4 and 5, indicative of metal-metal multiple bonding, and an unprecedented trigonal monopyramidal geometry about the Zr center in 5. Reduction of 4 under an Ar atmosphere (in the absence of N₂) results in another unusual structure type: an unoccupied axial Co coordination site and a trigonal monopyramidal Co center in (THF)Zr(MesNPⁱPr ₂)₃Co (6). X-ray crystallography reveals that, in the absence of coordinated N₂, the Co-Zr bond can attain full triple bond character with a Co-Zr distance of 2.14 A, the shortest M-M distance in an early/late heterobimetallic complex reported to date. To further assess the electronic structure and bonding in 4, 5, and 6, calculations were performed on these molecules using DFT and the results of these theoretical investigations will be discussed.

Full text of DOI:10.1021/ja909310t

Published on Web 12/14/2009
Metal-Metal Multiple Bonds in Early/Late Heterobimetallics Support Unusual
Trigonal Monopyramidal Geometries at both Zr and Co
Bennett P. Greenwood, Gerard T. Rowe, Chun-Hsing Chen, Bruce M. Foxman, and
Christine M. Thomas*
Department of Chemistry, Brandeis UniVersity, 415 South Street, Waltham, Massachusetts 02454
Received November 2, 2009; E-mail: thomasc@brandeis.edu
The highly polarized metal-metal bonds in early/late heterobi-
Scheme 1
metallic complexes present a unique strategy for tuning redox
1
potentials and small molecule activation processes. For example,
Casey and co-workers targeted early/late heterobimetallic dihydrides
featuring a hydridic early metal hydride and an acidic late metal
2
hydride as potential small molecule reducing agents. While a
number of early/late heterobimetallic complexes have been reported,
there are few accounts of reactivity in which both metals are
3
involved and limited reports of metal-metal multiple bonds
4
,5
between early and late transition metals. Wolczanski and co-
workers reported several alkoxyphosphine-linked heterobimetallic
complexes, namely Ti(OCMe
2 2 2 3 2
CH PPh ) Rh and Cp*Zr(OCH -
PPh RhMe , featuring triple and double M-Rh bonds, respec-
2
)
2
4
2
tively. In light of this, our group has chosen to incorporate more
reactive first row transition metals into new early/late heterobime-
tallic complexes. Herein, we report the synthesis of Co/Zr hetero-
bimetallic complexes featuring Co-Zr multiple bonds and the
ability of these metal-metal interactions to support coordinatively
unsaturated species, including an unprecedented trigonal monopy-
ramidal Zr complex.
i
i
adduct, Zr( PrNP Pr
2
)
3
2
Co-N (5, Scheme 1). In contrast to the
mesityl derivative, we were unable to isolate a NaX-bound product
similar to 3, presumably due to the lability of the NaX moiety.
The IR stretch corresponding to the bound N moiety of 5 was
2
2
2056 cm , showing a dramatic increase in the N vibrational
We previously reported the synthesis and characterization of a
7
family of Co/Zr heterobimetallic complexes linked by three
i
phosphinoamide ligands, including ICo(MesNP Pr
2
)
3
ZrCl (1) and
-1
i
i
6
2 3
ICo( PrNP Pr ) ZrCl (2). It was found that the dative metal-metal
frequency with respect to 4.
interactions in these complexes facilitated two-electron reduction
at potentials ∼1 V more positive than expected for analogous
monometallic complexes. Complex 1 was chemically reduced with
The structure of compound 5 is analogous to that of 4; however,
rather than binding THF, the axial coordination site of 5 remains
open (Figure 1). The geometry about Zr is trigonal monopyramidal,
excess Na/Hg, leading to the Co-bound dinitrogen product
8
i
an unprecedented geometry for Zr, with all three N-Zr-N bond
(
THF)
5
Na-X-Zr(NMesP Pr
2 3 2
) Co-N (3). Complex 3 shows a 0.22
angles within 0.1° of 120° (120.0°, 119.9°, and 120.0°) and
N-Zr-Co angles of 89.7°. The Zr-Co distance in 5 is moderately
shorter than that of 4 (avg. Zr-Co distance in three independent
molecules in asymmetric unit of 5 ) 2.33 Å). We attribute the
drastic decrease in backbonding from Co to N
by the increase in the ν(N
Å decrease in the distance between the two metal centers in
comparison with 1 (2.4112(3) Å vs 2.6280(5) Å), indicating a
substantial increase in Co-Zr bond order upon two-electron
reduction. Interestingly, the solid state structure revealed a strong
Na-X interaction and a weakly bound Zr halide resulting from a
trans-influence imparted on Zr by the electron-rich Co center,
suggesting the likelihood of Na-X extrusion from 3.
2
in 5, as indicated
) by 30 cm- with respect to 4, to stronger
2
1
involvement of the dxz and dyz orbitals in the Co-Zr bonding
interaction in the absence of sigma donation from THF to Zr.
As shown in Scheme 1, extraction of complex 3 into benzene
leads to precipitation of the labile NaX moiety, producing a red
i
diamagnetic complex (THF)Zr(MesNP Pr
2
)
3
2
Co-N (4), in which
a bound THF molecule occupies the axial coordination site on Zr.
Replacement of the NaX with THF leads to a subtle increase in
the ν(N
2
) from 2023 cm- in 3 to 2026 cm in 4. X-ray quality
1
-1
crystals of 4 were obtained, and the solid state structure is shown
in Figure 1. The Zr-Co distance in 4 is 2.36 Å (avg. of two
molecules in asymmetric unit), showing a slight contraction of the
Zr-Co bond compared to 3 (2.4112(3) Å) and a 0.27 Å decrease
Figure 1. Displacement ellipsoid (50%) representations of 4, 5, and 6.
Zr-Co distances of 4, 5, and 6 are 2.36(1), 2.33(1), and 2.14(1) Å,
respectively (averages of two (4, 6) or three (5) molecules in asymmetric
unit). All hydrogen atoms and the isopropyl methyl groups have been
omitted for clarity.
in the Co-Zr distance from the dihalide heterobimetallic precursor
6
1
(2.6280(5) Å).
Similarly, reduction of complex 2 with excess Na/Hg, followed
by subsequent benzene extraction, leads to a neutral dinitrogen
44
9 J. AM. CHEM. SOC. 2010, 132, 44–45
10.1021/ja909310t  2010 American Chemical Society

C O M M U N I C A T I O N S
Interestingly, we have found that reduction of 1 in an argon-
filled glovebox in the absence of N leads to an isolable diamagnetic
Co (6, Scheme 1). The solid
state structure reveals that 6 features an open Co coordination site
and a trigonal monopyramidal Co center (Figure 1). The absence
of an apical ligand at Co appears to dramatically strengthen the
bonding between Co and Zr: the Co-Zr bond length contracts to
hypothesis, the electronic structure of 6, the dinitrogen-free analogue
of 4, was determined (Figure 2B). Particularly striking in the frontier
orbitals of 6 is the degree to which the three lowest energy orbitals
exhibit Co-Zr bonding interactions, clearly forming one σ and two
π bonds between the metal centers. This increase in M-M multiple
bonding character is reflected in the contraction of the Co-Zr
distance by ca. 0.2 Å between 4 and 6.
2
i
2 3
green product, (THF)Zr(MesNP Pr )
2
.14 Å.
To assess bond distances while taking into account differences
in atomic size, Cotton defined the “formal shortness ratio”: FSRAB
A
B
)
D
AB/R
1
+ R
1
, where DAB is the interatomic distance and the
R
1
values are the van der Waal’s radii of the two atoms (Co: 1.152
10
Å, Zr: 1.454 Å). By this treatment, we find the FSR of complexes
, 5, and 6 to be 0.902, 0.891, and 0.818, respectively. For
4
6
comparison, the FSR of complexes 1 and 2 are 1.00. Thus, complex
6
unequivocally holds the record for the shortest M-M distance
in an early/late heterobimetallic complex in both absolute distance
8
and FSR, greatly surpassing the Ti-Rh complex of Wolczanksi
4
a
et al. (FSR ) 0.860) and the Mo-Ru complexes of Collman et
5
a
al. (FSR ) 0.869, 0.857).
In summary, we have synthesized three heterobimetallic com-
plexes featuring Co-Zr multiple bonds, including two complexes
featuring unusual trigonal monopyramidal geometries at either the
Zr (5) or Co (6) center. We anticipate that these highly reduced,
coordinatively unsaturated molecules will be reactive toward a wide
variety of small molecule substrates and future works will focus
on their reactivity.
Acknowledgment. This work was supported by Brandeis
University.
Supporting Information Available: Experimental details, crystal-
lographic data for complexes 4, 5, and 6 (in CIF format) and additional
computational details. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 2. Depictions and energies of the frontier molecular orbitals of
complexes 5 (A) and 6 (B).
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