Phosphorus phenyl-group activation by reduced zirconium and niobium complexes stabilized by the [P2N2] macrocycle

Article abstract of DOI:10.1021/ja012100q

Reduction of [P₂N₂]ZrCl₂ (where [P₂N₂] = PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh) with KC₈ under argon generates the phosphorus phenyl bridged bimetallic complex where the bridging phenyl groups are formally reduced to bis(allyl) dianions. Similar reduction of [P₂N₂]NbCl caused the one-electron reduction of the phosphorus phenyl group to generate a cyclohexadienyl moiety via a C-C bond formation between the ipso carbons of the two phenyl groups. Copyright

Full text of DOI:10.1021/ja012100q

Published on Web 01/05/2002
Phosphorus Phenyl-Group Activation by Reduced Zirconium and Niobium
Complexes Stabilized by the [P2N2] Macrocycle
Michael D. Fryzuk,* Christopher M. Kozak, Parisa Mehrkhodavandi, Lara Morello,
Brian O. Patrick,† and Steven J. Rettig†,‡
Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall,
VancouVer, British Columbia V6T 1Z1, Canada
Received September 4, 2001
The activation of dinitrogen by transition metal complexes has
been an active area of research for almost four decades.1-3 Despite
the considerable investment of effort and time, the synthesis of
complexes that contain coordinated dinitrogen is still considered
difficult and oftentimes fortuitous. The most widely used method
is that of reduction whereby a metal complex in a relatively high
oxidation state is combined with a strong reducing agent in the
presence of dinitrogen. Depending on the ligands, the metal, the
solvent and the reducing agent, sometimes a dinitrogen complex
results. As part of our general program aimed at a better
understanding of the mode of formation of dinitrogen complexes,
Figure 1. Molecular structure of ([P2N2]Zr)2, 1. Silyl methyl groups are
we have been examining the reaction of group 4 and 5 transition
metal complexes with reducing agents.4,5 As already reported6
reduction of [P2N2]ZrCl2 (where [P2N2] ) PhP(CH2SiMe2NSiMe2-
CH2)2PPh) under 4 atm of N2 with potassium graphite (KC8) results
in the formation of the side-on bridged dinitrogen complex ([P2N2]-
Zr)2(µ-η2:η2-N2); similarly, reduction of [P2N2]NbCl7 in the presence
of N2 with KC8 generates the end-on derivative, ([P2N2]Nb)2(µ-
N2).8 Interestingly, when these reductions are performed in the
absence of dinitrogen, phosphorus-phenyl activation occurs to
generate highly activated arene-bridged derivatives. In this work
we describe the preparation of two new bimetallic complexes in
which arene activation occurs when dinitrogen is excluded.
During the preparation of dark blue ([P2N2]Zr)2(µ-η2:η2-N2), a
small amount (<5%) of a yellow crystalline byproduct was
sometimes observed. A Pasteur separation of a few crystals of this
material allowed analysis by X-ray crystallography; the molecular
structure and numbering scheme of the material is shown in Figure
1 along with selected bond lengths and bond angles.9 What became
evident is that this byproduct does not contain dinitrogen; instead,
a phosphorus-phenyl group on one [P2N2]Zr unit is bound to the
zirconium center of another [P2N2]Zr fragment.10 When this
reduction was performed in the absence of N2 (either under argon
or in a vacuum), good yields of ([P2N2]Zr)2 (1) were obtained (eq
1).11 The NMR spectroscopic parameters of 1 are consistent with
this formulation, as are elemental analyses and mass spectrometric
data. The 1H NMR spectrum shows one set of phosphorus-phenyl
resonances in the ratio 2:2:1 upfield shifted to 4.35, 4.20, and 3.30
ppm indicative of coordination. The 31P{1H} NMR spectrum is a
AA′BB′ pattern due to coupling information being transmitted
through the coordinated arene ring between metal centers.
omitted for clarity and only ipso carbons of phenyl rings are shown. Selected
bond lengths (Å), angles (deg): Zr1-C13*, 2.352(3); Zr1-C14*, 2.460-
(3); Zr1-C15*, 2.542(3); Zr1-C16*, 2.401(3); Zr1-C17*, 2.528(3); Zr1-
C18*, 2.551(3); Zr(1)-CM, 2.03; C13-C14, 1.488(5); C14-C15, 1.384(5);
C15-C16, 1.399(5); C16-C17, 1.447(5); C17-C18, 1.359(5); C13-C18,
1.439(4); P1-Zr1-CM, 109.3.
conformation with a dihedral angle of 29.9° between the planes
defined by C13-C14-C18 and C15-C16-C17. The allyl units
on each activated P-phenyl group correspond to C14-C15-C16
and C17-C18-C13 on the basis of shorter C-C bond lengths in
the range of 1.359(5) to 1.439(4) Å. The C13-C14 and C16-C17
bond lengths of 1.488(5) and 1.447(5) Å, respectively, are longer
and approximate the single C-C bonds that connect the two allyl
moieties within each ring.
In the analogous high-yield preparation of the paramagnetic, end-
on niobium dinitrogen complex, ([P2N2]Nb)2(µ-N2), similar byprod-
ucts are not observed. However, if reduction of [P2N2]NbCl is
performed in the absence of dinitrogen, a pale green, diamagnetic
solid is obtained in good yield (eq 2). In addition to resonances
from the [P2N2] macrocycle, the 1H NMR spectrum displays three
upfield-shifted phosphorus-phenyl resonances with 1:2:2 integration
at 5.24, 3.99, and 3.58 ppm suggesting a coordinated arene
ligand.13,14 The 31P{1H} NMR spectrum consists of a broad pair of
doublets at 44.94 and 18.16 ppm, suggesting coupling between two
inequivalent phosphorus environments; any second-order effects
are masked by the quadrupolar niobium center. If one assumes that
The solid state structural data show that the bridging phosphorus-
phenyl rings are highly distorted and bind in a η3:η3 fashion. The
bonding in this complex can be viewed as the interaction of a Zr-
(IV) fragment with the phenyl ring acting as a bis(allyl) dianion.12
The coordinated phenyl ring is no longer planar and adopts a “boat”
† UBC X-ray Structural Chemistry Laboratory.
‡ S.J.R. deceased October 27, 1998.
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10.1021/ja012100q CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
The two systems described above illustrate the complexity that
can result by the use of strong reducing agents in the preparation
of dinitrogen complexes. Because dinitrogen is not a particularly
good ligand,24 highly reduced complexes can undergo other
competing reactions rather than activate N2. In the zirconium case,
this is certainly true, although higher pressures of dinitrogen can
minimize the formation of dimer 1. For the niobium analogue, dimer
2 is not observed even when the reduction of [P2N2]NbCl is carried
out at 1 atm pressure of N2; nevertheless, the similarity between
the two bimetallic species is remarkable and suggests that similar
but as yet unknown mechanisms are operative in these processes.
Figure 2. Molecular structure of ([P2N2]Nb)2, 2. Silyl methyl groups are
omitted for clarity and only ipso carbons of phenyl rings are shown. Selected
bond lengths (Å), angles (deg): Nb1-C20*, 2.313(3); Nb1-C(21)*, 2.367-
(3); Nb1-C22*, 2.415(3); Nb1-C23*, 2.390(3); Nb1-C24*, 2.340(3);
Nb1-CM, 1.925; C19-C19*, 1.542(5); C19-C20, 1.5294; C20-C21,
1.429(4); C21-C22, 1.408(4); C22-C23, 1.396(4); C23-C24, 1.427(4);
C24-C19, 1.520(4); Nb1-CM-C22*, 101.29; P2-C19-C19*, 98.4(2).
Acknowledgment. We thank both NSERC of Canada and the
Petroleum Research Fund, administered by the American Chemical
Society, for funding in the form of research grants.
Supporting Information Available: Experimental details and
spectroscopic and X-ray structural data for 1 and 2 (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
the structure of niobium dimer 2 is identical to that of zirconium
dimer 1, then the diamagnetism of the former is not easily
reconciled; the two niobium(IV) centers would have to be strongly
antiferromagnetically coupled to produce a diamagnetic ground
state.
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(9) Crystal data for 1: [C48H84N4P4Si8Zr2 (fw ) 1248.24)], brown block,
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Supporting Information.
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analysis; the numbering scheme and ORTEP are shown in Figure
2 along with selected bond lengths and bond angles.9 The structure
shows that a phosphorus-phenyl ring of one [P2N2]Nb unit is
coordinated to another niobium center, analogous to that found in
1. However, there are major differences in arrangement of the
bridging arene units. The niobium complex undergoes a one-
electron reduction of the P-phenyl ring to generate a cyclohexadi-
enyl moiety via C-C bond formation between the ipso carbons of
the two phenyl groups. Thus, each P-phenyl ring binds in a η5-
cyclohexadienyl fashion. The C19-C19* bond length of 1.542(5)
Å is consistent with single bond character. In the niobium complex,
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of each metal in 2 can be described as low-spin Nb(III). These
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has sometimes been described as a neutral donor.
(10) For a similar bonding mode in a Zr complex see: Cotton, F. A.; Wojtczak,
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(11) A byproduct of this reaction is the toluene adduct [P2N2]Zr(η3:η3-C7H8)
usually evident in 5-15% yield.
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R.; Arnold, J. Angew. Chem., Int. Ed. Engl. 1998, 37, 1729. (b) Cassani,
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such as H2 and N2. This suggests that in neither case are these
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