Formation of phosphorus-nitrogen bonds by reduction of a titanium phosphine complex under molecular nitrogen

Article abstract of DOI:10.1021/ja054467r

The reduction of high oxidation state metal complexes in the presence of molecular nitrogen is one of the most common methods to synthesize a dinitrogen complex. However, the presence of strong reducing agents combined with the poor binding ability of N₂ can lead to unanticipated outcomes. For example, the reduction of [NPN]ZrCl₂(THF) (where NPN = PhP(CH₂SiMe₂NPh)₂) with KC₈ under N₂ leads to the formation of the side-on bridged dinuclear dinitrogen complex ([NPN]Zr(THF))₂(μ-η²:η²-N₂) with an N-N bond distance of 1.503(3) A; however, reduction of the corresponding titanium precursor, [NPN]TiCl₂, under N₂ does not generate a dinitrogen complex, rather the bis(phosphinimide) derivative, ([N(PN)N]Ti)₂, is isolated in which the added N₂ is incorporated between the titanium and phosphine centers. Performing the reaction under ¹⁵N₂ results in the ¹⁵N label being incorporated in the phosphinimide unit. A suggested mechanism for this process involves an initially formed dinitrogen complex being over reduced to generate a species with bridging nitrides that undergoes nucleophilic attack by the coordinated phosphine ligands and formation of the P=N bond of the phosphinimide. Copyright

Full text of DOI:10.1021/ja054467r

Published on Web 08/25/2005
Formation of Phosphorus-Nitrogen Bonds by Reduction of a Titanium
Phosphine Complex under Molecular Nitrogen
Lara Morello, Peihua Yu, Christopher D. Carmichael, Brian O. Patrick, and Michael D. Fryzuk*
Department of Chemistry, The UniVersity of British Columbia, 2036 Main Mall, VancouVer, BC, Canada, V6T 1Z1
Received July 6, 2005; E-mail: fryzuk@chem.ubc.ca
A very common synthetic method to generate dinitrogen metal
complexes is via reduction, wherein addition of some alkali or
alkaline earth metal (e.g., Li, Na/Hg, KC8, Mg) to a high oxidation
state, precursor metal complex in the presence of N2 can result in
dinitrogen binding.1 Only in a few cases have lower oxidation state
complexes been isolated or detected and shown to react indepen-
dently with dinitrogen.2 In most instances, little is known about
intermediates and how the N2 interacts with the metal. As a result,
these reduction protocols are not easily controlled because the
combination of strong reducing agents and poor binding ability of
molecular nitrogen can result in unpredictable outcomes.3 In this
work, we detail the effect of a change of the metal center within a
particular ligand during such a reduction process; what results is
an unanticipated process that involves intramolecular phosphorus-
nitrogen bond formation, where the nitrogen originates from added
N2.
Our recent efforts have been directed at the synthesis and
reactivity of well-defined dinitrogen complexes. Within group 4,
we have previously reported4 the synthesis of dinuclear dinitrogen
Figure 1. ORTEP diagram of complex 1. The ellipsoids are drawn at the
50% level. Selected bond distances (Å) are: N3-N3*, 1.503(3); Zr1-N3,
complexes with side-on bridging N2 units stabilized by ancillary
ligands that contain phosphine and amido donors. In the case of
the reduction of [P2N2]ZrCl2 (where [P2N2] ) PhP(CH2SiMe2-
NSiMe2CH2)2PPh) with KC8 under N2, ([P2N2]Zr)2(µ-η2:η2-N2) is
generated in good yields if the N2 pressure is maintained at 4 atm;
lower pressures of N2 can result in contamination by a P-phenyl
bridged dimer.5
Reduction of the related Zr(IV) precursor complex [NPN]ZrCl2-
(THF) (where [NPN] ) PhP(CH2SiMe2NPh)2) with KC8 in THF
under N2 generates ([NPN]Zr(THF))2(µ-η2:η2-N2) (1) in very good
yields (eq 1).
2.026(2); Zr1-N3*, 2.069(2); Zr1-N1, 2.175(2); Zr1-N2, 2.228(2); Zr1-
O1, 2.305(1); Zr1-P1, 2.6685(5). Silylmethyl groups are omitted for clarity;
only ipso carbons of the phenyl groups are shown.
ancillary ligand backbone. The solid-state structure is shown in
Figure 1. The dinuclear complex displays a side-on bound N2 ligand
with a long N-N bond of 1.503(3) Å consistent with an N-N
single bond and a N24- unit. There are a number of side-on bridged
dizirconium dinitrogen complexes that display similar levels of
activation as evidenced by long N-N bond lengths.4,7
Reduction of the Ti(IV) precursor [P2N2]TiCl2 with activated
KC8 generates ([P2N2]Ti)2(µ-N2);6 the bridging N2 unit is end-on
bound to each Ti center, and the N-N bond length is 1.255(7) Å.
However, when the corresponding diamidophosphine precursor,
[NPN]TiCl2, is utilized, a different outcome is obtained.
The reaction of [NPN]TiCl2 with 2 equiv of KC8 under dinitrogen
in THF results in a series of color changes from the initial orange
of the starting material to generate an olive green solution from
which emerald green crystals could be obtained. The elemental
analysis and mass spectral data are consistent6 with a molecular
formula that involves incorporation of 1 equiv of N2, that is, ([NPN]-
Ti)2(N2) 2 (eq 2). The complex is diamagnetic and displays a singlet
in the 31P{1H} NMR spectrum at δ 39.9, considerably downfield
1
of the starting dichloride (δ 3.7); the resonances in the H NMR
spectrum are consistent with a molecule of high symmetry as only
two silylmethyl peaks are observed, and the methylene protons
appear as an ABX-type pattern. With the assumption that this was
a bona fide dinitrogen complex, all that remained was to determine
the bonding mode of the N2 unit, either end-on bridging in analogy
to ([P2N2]Ti)2(µ-N2) described above, or side-on bridging similar
to that found for the zirconium derivative 1.
This complex was characterized6 by solution NMR spectroscopy
and solid-state X-ray crystallography. The overall high symmetry
of 1 in solution is evident by the observation of a singlet in the 31
P
1
NMR spectrum at δ -5.6 along with resonances in the H NMR
spectrum consistent with one kind of coordinated THF environment
along with appropriate signals for the remaining protons of the
The structure of 2 as determined by X-ray crystallography is
shown in Figure 2. What is clearly evident is that there is no intact
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J. AM. CHEM. SOC. 2005, 127, 12796-12797
10.1021/ja054467r CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
dinitrogen moiety coordinated to the dititanium core. In fact, the
dinuclear metal complex has only the [NPN] bound but with one
important change: the phosphine donor has been transformed into
a phosphinimide unit.8 Formally, the [N(PN)N] ligand set (where
[N(PN)N] ) (PhNSiMe2CH2)2P(dN)Ph) is a trianion, and thus the
formal oxidation state of each titanium center in ([N(PN)N]Ti)2
(2) is Ti(III). The Ti-Ti bond distance is 2.6710(6) Å and is bridged
by the imido unit of the phosphinimide portion of the ancillary
ligand; the remaining sites are occupied by the phenylamido units
of the original ligand set. The P-N bond distance of 1.591(2) Å is
not unusual, being well in the range of reported bond lengths for
this unit.9
The origin of the nitrogen atoms that have been inserted between
the phosphine and the titanium centers was investigated by reduction
under 15N2, which produced ([N(P15N)N]Ti)2 (15N-2) as evidenced
by mass spectrometry and confirmed by 31P NMR spectroscopy
(δ 39.9, d, 1J15N ) 22 Hz). This clearly indicates that the source of
the nitrogen of the phosphinimide unit is added N2.
Shown in Scheme 1 is a possible mechanism for the cleavage
of coordinated N2 and insertion into the Ti-P bond. There is no
precedent for direct attack of a phosphine on a coordinated N2
moiety. However, it is presumed that some dinuclear N2 complex
initially forms, which is subsequently reduced further to generate
the dianionic bridging dinitride species B. The transformation of
B to C involves nucleophilic attack of a phosphine to the bridging
nitride, a process that has precedent in nitride coupling reactions
and in the reaction of PPh3 with Os(VI) nitrides10 to generate Os-
NPPh3 species in which reduction to Os(IV) has taken place. The
transformation of the dianion C to product 2 requires release of
two electrons, making the over reduction of A electrocatalytic.
The zirconium congener 1 does not undergo this reaction under
any conditions that we have tried. It is likely the ease of reduction
of Ti(IV) in 2 versus Zr(IV) in 1 that facilitates this new
transformation.
This study reinforces the notion of the lack of control that is
possible in using strong reducing agents as a method to incorporate
molecular N2 into the coordination sphere of metal complexes.
Cleavage of the N-N bond in N2 complexes to form metal nitrides
is well documented in the literature11 as is the reaction of
nucleophiles with high oxidation state nitrides;10 the combination
of these two processes leads to ancillary ligand modification by
the formation of a phosphinimide functionality facilitated by the
over reduction of the initially formed N2 complex.
Acknowledgment. This research was supported by the Natural
Sciences and Engineering Research Council (NSERC) of Canada.
Supporting Information Available: Experimental details for all
of the compounds reported. Details on the solution and refinement of
X-ray crystal structures. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Figure 2. ORTEP diagram of complex 2. The ellipsoids are drawn at the
50% probability level. Selected bond distances (Å) are: Ti1-Ti2, 2.6710-
(6); Ti1-N5, 1.997(2); Ti1-N6, 1.980(2); Ti2-N5, 1.978(2); Ti2-N6,
1.993(2); Ti1-N1, 2.042(2); Ti1-N2, 2.008(2); Ti2-N3, 1.992(2); Ti2-
N4, 2.046(2); P1-N5, 1.591(2); P2-N6, 1.590(2). Silylmethyl groups are
omitted for clarity; only ipso carbons of the phenyl groups are shown.
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J. AM. CHEM. SOC. VOL. 127, NO. 37, 2005 12797