Synthesis and Structure of a Zirconium Dinitrogen Complex with a Side-On Bridging N2 Unit

Article abstract of DOI:10.1021/ic9709786

Reduction of Zr(O-2,6-Me₂C₆H₃)Cl₂[N(SiMe ₂CH₂PPrⁱ₂)₂] with sodium amalgam under dinitrogen yields the dinuclear zirconium dinitrogen complex {[(Pr₂ⁱPCH₂SiMe₂) ₂N]Zr(O-2,6-Me2C6H3)}2(/<-?72:??2-N2). Solid state structural analysis shows that the dinitrogen unit is bound in a side-on mode of coordination with the N-N bond distance at 1.528(7) A?; resonance Raman spectra show a band at 751 cim^-1 for ν(N-N), which is consistent with this very long bond. In addition, the N₂ ligand is hinged slightly so that the Zr₂(μ-η₂:η₂-N₂) core adopts a flattened butterfly shape rather than a completely planar core as found in other related systems. Other Zr(IV) precursors of the general formula ZrCl₂X[N(SiMe₂CH₂PPr₂ ⁱ)₂] (X = OBu^t, OCHPh₂, NPh₂) either decompose upon reduction under N₂ or produce mixtures of products.

Full text of DOI:10.1021/ic9709786

112
Inorg. Chem. 1998, 37, 112-119
Synthesis and Structure of a Zirconium Dinitrogen Complex with a Side-On Bridging N2
Unit
Jonathan D. Cohen,† Michael D. Fryzuk,*,‡ Thomas M. Loehr,*,†
Murugesapillai Mylvaganam,‡ and Steven J. Rettig‡,§
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, BC, Canada V6T 1Z1, and Department of Biochemistry and Molecular Biology,
Oregon Graduate Institute of Science and Technology, Portland, Oregon 97291-1000
ReceiVed August 7, 1997X
Reduction of Zr(O-2,6-Me2C6H3)Cl2[N(SiMe2CH2PPri2)2] with sodium amalgam under dinitrogen yields the
dinuclear zirconium dinitrogen complex {[(Pri2PCH2SiMe2)2N]Zr(O-2,6-Me2C6H3)}2(µ-η2:η2-N2). Solid state
structural analysis shows that the dinitrogen unit is bound in a side-on mode of coordination with the N-N bond
distance at 1.528(7) Å; resonance Raman spectra show a band at 751 cm-1 for ν(N-N), which is consistent with
this very long bond. In addition, the N2 ligand is hinged slightly so that the Zr2(µ-η2:η2-N2) core adopts a flattened
butterfly shape rather than a completely planar core as found in other related systems. Other Zr(IV) precursors
of the general formula ZrCl2X[N(SiMe2CH2PPri2)2] (X ) OBut, OCHPh2, NPh2) either decompose upon reduction
under N2 or produce mixtures of products.
Introduction
dinitrogen by Mo(III) to form a molybdenum nitride species
has been reported.17,18 Recently, we described the preparation
and structures of dinuclear complexes of zirconium that contain
dinitrogen in either the side-on bridging or the end-on bridging
mode of ligation.19-21 Thus, {[(Pri2PCH2SiMe2)2N]ZrCl}2(µ-
η2:η2-N2) (1) has the N2 unit side-on with a N-N bond length
“The fixation of dinitrogen is one of the great discoveries
awaiting the ingenuity of chemists.” These words1 by William
Crookes in 1898 foreshadowed the great discovery in 1905 of
the Haber process,2,3 which is presently used to produce millions
of tons of ammonia from dinitrogen and dihydrogen. Neverthe-
less, for the coordination chemist, the challenge4 still remains
to activate N2 under conditions milder than those typically used
by BASF in the Haber-Bosch version5 of this process.
Although the coordination chemistry of the N2 molecule has
flourished under this challenge and much new fundamental
knowledge has been gleaned,6-12 no reaction comparable to the
Haber discovery has yet been uncovered.
Recent fundamental studies on the binding of N2 to metal
complexes have shown some remarkable developments.12 Not
only do transition metals bind N2 in a variety of bonding
modes,11 but complexation to lithium,13 gold,14 and samari-
um15,16 has also been achieved. In addition, cleavage of
of 1.548(7) Å, while {[(Pri2PCH2SiMe2)2N]Zr(η5-Cp)}2(µ-N2)
(2) has the dinitrogen ligand end-on with a N-N bond distance
of 1.301(3) Å. By changing the ancillary ligand from a
tridentate amidodiphosphine to a macrocyclic system containing
two amido units, we were also able to prepare the side-on
dinitrogen complex {[(PhP(CH2SiMe2NSiMe2CH2)2PPh]Zr}2-
(µ-η2:η2-N2) (3); the N-N bond length is 1.43(4) Å, and the
complex displays new modes of reactivity with H2 and silanes.19
We have also suggested20 that the frontier orbitals of the
zirconium center could be influenced by the ancillary ligands
and matched to the π* orbitals of the N2 fragment in either the
side-on or the end-on mode. Because of our continuing interest
in fundamental studies of dinitrogen coordination, we embarked
on a program to examine the effects of different ligand
environments on the preparation and structure of zirconium
dinitrogen complexes. In this paper, we summarize our efforts
and provide another example of a side-on bound dinitrogen
complex.
† Oregon Graduate Institute of Science and Technology.
‡ University of British Columbia.
§ Experimental Officer, UBC Structural Chemistry Laboratory.
X Abstract published in AdVance ACS Abstracts, December 15, 1997.
(1) Crookes, W. The Report of the 68th Meeting of the British Association
for the AdVancement of Science, Bristol; John Murray: London, 1898.
(2) Feldman, M. R.; Tarver, M. L. J. Chem. Educ. 1983, 60, 463.
(3) Haber, F.; van Oordt, G. Z. Anorg. Chem. 1905, 44, 341.
(4) Green, M. L. H. J. Chem. Soc., Dalton Trans. 1991, 575.
(5) Jennings, J. R. Catalytic Ammonia Synthesis: Fundamentals and
Practice; Plenum Press: New York and London, 1991; Vol. 1.
(6) Sellmann, D. Angew. Chem., Int. Ed. Engl. 1974, 13, 639.
(7) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. ReV. 1978, 78, 589.
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Radiochem. 1983, 27, 197.
(15) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1988,
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(17) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861.
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E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem.
Soc. 1996, 118, 8623.
(19) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997,
275, 1445.
(20) Fryzuk, M. D.; Haddad, T. S.; Mylvaganam, M.; McConville, D. H.;
Rettig, S. J. J. Am. Chem. Soc. 1993, 115, 2782.
(21) Cohen, J. D.; Mylvaganam, M.; Fryzuk, M. D.; Loehr, T. M. J. Am.
Chem. Soc. 1994, 116, 9529.
(9) Pelikan, P.; Boca, R. Coord. Chem. ReV. 1984, 55, 55.
(10) Leigh, G. J. Acc. Chem. Res. 1992, 25, 177.
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(12) Gambarotta, S. J. Organomet. Chem. 1995, 500, 117.
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3792.
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S0020-1669(97)00978-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/12/1998

A Zirconium Dinitrogen Complex
Inorganic Chemistry, Vol. 37, No. 1, 1998 113
wavenumber assignments were estimated to be accurate within 2 cm-1
.
Spectra for polarization studies of solution samples were collected in
a 90° scattering geometry at 278 K on a Dilor Z-24 Raman spectro-
photometer. Typical slit-width settings were between 5 and 9 cm-1
.
All samples were sealed under N2 in 1.5-1.8 × 90 mm Kimax glass
capillary tubes.
UV-vis spectra were recorded on a Perkin Elmer 5523 UV/Vis
spectrophotometer stabilized at 20 °C. Mass spectral studies were
carried out on a Kratos MS 50 using an EI source. IR spectra were
recorded on a Bomem MB-100 spectrometer. Solution samples were
recorded in a 0.1 mm KBr cell, and solid samples were recorded as
KBr pellets. Carbon, hydrogen, and nitrogen analyses were performed
by the microanalyst of the UBC Chemistry Department.
The starting complex ZrCl3[N(SiMe2CH2PPri2)2] was prepared ac-
cording to the published procedure.22 Mercury was purchased from
BDH and purified as follows: In a separatory funnel, mercury (500 g)
was washed with 2 M HCl (2 × 25 mL) acid and then with distilled
water (2 × 50 mL), and it was finally rinsed a few times with Et2O
(25 mL) until no further gray color was present in the ether washings.
During washings, a slag was formed on the surface of the Hg which
was separated from the shiny Hg. Pure Hg was dried under vacuum
for 12 h. Sodium amalgam was made under a nitrogen atmosphere
and washed with toluene (2 × 25 mL) until the washings showed no
gray coloration.
The sodium alkoxide and aryloxide reagents, NaOCHPh2 and NaO-
2,6-Me2C6H3, were prepared by reacting a toluene solution of the
alcohol with sodium (3 h at room temperature (RT) followed by a 3 h
reflux). The resulting white solid was filtered off and extracted with
THF (to remove finely dispersed sodium), and the THF was removed
under vacuum to give alcohol-free NaOR. Prior to the reaction, the
alcohol was dissolved in toluene, and the solution was stirred with Mg
turnings (0.1 equiv) for 12 h, and the mixture was filtered. KOBut
was purchased from Aldrich and was sublimed prior to use. The sodium
amide, NaNPh2, was prepared by reacting the amine, HNPh2, with NaN-
(SiMe3)2 in toluene. The NaNPh2 was precipitated out of toluene,
collected on a frit, and washed with hexanes to obtain pure material.
Zr(O-2,6-Me2C6H3)Cl2[N(SiMe2CH2PPri2)2], 5. To a solution of
ZrCl3[N(SiMe2CH2PPri2)2], 4 (4.00 g, 5.08 mmol), in toluene (150 mL)
was added solid Na(O-2,6-Me2C6H3) (1.03 g, 6.35 mmol) in three
portions at 1 h intervals at RT. The reaction mixture was stirred for
16 h, and then the salt (NaCl) was removed by filtering through Celite.
The filtrate was concentrated to 25 mL, an equal volume of hexanes
added, and the mixture allowed to stand at room temperature for 24 h.
A colorless crystalline product slowly separated from the solution (2.41
g, 70%). 1H NMR (δ, 300 MHz, C6D6): 0.48 (s, 12H, Si(CH3)2); 0.96
Experimental Section
General Procedures. All manipulations were performed under
prepurified nitrogen in a Vacuum Atmospheres HE-553-2 work station
equipped with an MO-40-2H purification system or in Schlenk-type
glassware. Pentane, diethyl ether, and hexanes were dried and
deoxygenated by distillation from sodium-benzophenone ketyl under
argon. Toluene was predried by refluxing over CaH2 and then distilled
from sodium under argon. Tetrahydrofuran and hexanes were predried
by refluxing over CaH2 and then distilled from sodium-benzophenone
ketyl under argon. Deuterated benzene (C6D6, 99.6 atom % D) and
deuterated toluene (C7D8, 99.6 atom % D), purchased from MSD
Isotopes, were dried over activated 4-Å molecular sieves, vacuum-
transferred, and freeze-pump-thawed three times before use.
3
(d of d, 16H, 12H of P[CH(CH3)2]2 and 4H of SiCH2P, JH-H ) 6.1
3
3
Hz, JP-H ) 14.0 Hz); 1.06 (d of d, 12H, P[CH(CH3)2]2, JH-H ) 6.1
Hz, 3JP-H ) 14.0 Hz); 2.00 (sept of t, 4H, P[CH(CH3)2]2, 3JH-H ) 6.1
Hz, 2JP-H ) 2.0 Hz); 2.80 (s, 6H, 2, 6-Me2Ph); 6.82 (t, 1H, p-Ph, 3JH-H
3
Dinitrogen gas was purified by purging through a column containing
MnO and activated 4-Å molecular sieves. The 99% 15N2 was obtained
from Cambridge Isotopes Ltd. and was used as supplied.
) 7.4 Hz); 7.05 (d, 2H, m-Ph, JH-H ) 7.4 Hz). 31P{1H} NMR (δ,
121.421 MHz, C6D6): 14.98 (s). 13C{1H} NMR (δ, 50.323 MHz,
C6D6): 5.41 (s, Si(CH3)2); 10.70 (s, SiCH2P); 19.07 (s, P[CH(CH3)2]2);
19.15 (s, P[CH(CH3)2]2); 19.86 (s, 2,6-Me2Ph); 24.09 (t, P[CH(CH3)2]2,
2JC-P ) 6.2 Hz); 120.94 (s, p-Ph); 128.79 (s, m-Ph). Anal. Calcd for
C26H53Cl2NOP2Si2Zr: C, 46.20; H, 7.90; N, 2.07. Found: C, 46.07;
H, 8.10; N, 2.03.
1
The H, 31P, and 13C NMR spectra were recorded on a Varian XL-
300, a Bruker AC 200, a Bruker WH-400, or a Bruker AM 500
spectrometer. Proton spectra were referenced using the partially
deuterated solvent peak as the internal reference, C6D5H at 7.15 ppm
and C6D5CD2H at 2.09 ppm relative to Me4Si. The 31P{1H} NMR
spectra were referenced to external P(OMe)3 set at +141.00 ppm relative
to 85% H3PO4. The 13C{1H} NMR spectra were referenced to the C6D6
signal at 128.0 ppm or to the CD3C6D5 signal at 20.4 ppm. Solution
15N{1H} NMR spectra were recorded on the Varian XL-300, referenced
to external formamide set at 0.00 ppm. Solid-state 15N NMR spectra
were run on a Bruker MSL-400, referenced to solid NH4Cl set at -73.39
ppm with respect to neat formamide at 0.00 ppm. 1H{31P} NMR spectra
were recorded on the Bruker AM 500 spectrometer.
Zr(OBut)Cl2[N(SiMe2CH2PPri2)2], 6. To a solution of ZrCl3-
[N(SiMe2CH2PPri2)2] (1.50 g, 2.54 mmol) in Et2O (60 mL) was added
a solution of KOBut (285 mg, 2.54 mmol) in Et2O (10 mL) at RT, and
the mixture was stirred for 3 h. The solvent was stripped off under
vacuum the residues were extracted with pentane (40 mL), and the
extracts were filtered through a layer of Celite. Stripping off the solvent
1
gave a colorless oil containing >80% (by H NMR spectroscopy) of
the desired product. 1H NMR (δ, 300 MHz, C6D6): 0.45 (s, 12H, Si-
(CH3)2); 1.05 (d, 4H, SiCH2P, 2JP-H ) 5.7 Hz); 1.32 and 1.35 (each d
Resonance Raman spectra21 were recorded on a modified Jarrell-
Ash model 25-300 Raman spectrometer. Excitation radiation was
supplied by Spectra Physics Ar+ and Kr+ lasers operating at 514.5 and
647.1 nm, respectively. Raman light was collected in a backscattering
geometry. Multiple scans were collected and calibrated against indene,
toluene, or THF as external standards for peak positions. All
3
3
of d, 24H, P[CH(CH3)2]2, JP-H ) 2.9 Hz, JH-H ) 7.6 Hz); 1.50 (s,
2
9H, OC(CH3)3); 2.04 (t of sept, 4H, P[CH(CH3)2]2, JP-H ) 1.9 Hz,
3JH-H ) 7.6 Hz). 31P{1H} NMR (δ, 81.015 MHz, C6D6): 11.60 (s).
(22) Fryzuk, M. D.; Carter, A.; Westerhaus, A. Inorg. Chem. 1985, 24,
642.

114 Inorganic Chemistry, Vol. 37, No. 1, 1998
Table 1. 1H{31P} NMR Data for Complexes 9 and 9a
Cohen et al.
groups
major isomer (δ)
minor isomer (δ)
0.29 (s, 6H), 0.32 (s. 6H)
Si(CH3)2
0.37 (s, 12H)
0.41 (s, 12H)
0.32 (s, 6H), 0.35 (s, 6H)
obscured
SiCH2P
1.30 (d, 4H, 2JH-H ) 1.6 Hz)
1.32 (d, 4H, 2JH-H ) 1.6 Hz)
1.04 (d, 12H, 3JH-H ) 6.7 Hz)
1.05 (d, 12H, 3JH-H ) 6.7 Hz)
1.17 (d, 12H, 3JH-H ) 7.3 Hz)
1.23 (d, 12H, 3JH-H ) 7.3 Hz)
2.03 (sept, 4H, 3JH-H ) 7.3 Hz)
2.39 (sept, 4H, 3JH-H ) 6.7 Hz)
P[CH(CH3)2]2
1.13 (d, 6H), 1.26 (d, 6H)
1.34 (d, 6H), 1.44 (d, 6H)
rest of the resonances were obscured
P[CH(CH3)2]2
2.04 (sept, 2H, 3JH-H ) 7.4 Hz)
2.20 (sept, 2H, 3JH-H ) 7.4 Hz)
2.48 (sept, 2H, 3JH-H ) 7.4 Hz)
2.53 (sept, 2H, 3JH-H ) 7.4 Hz)
2,6-Me2Ph
p-Ph
m-Ph
2.34 (s, 12H)
only assignable resonance was a broad peak at 2.32
6.66 (t, 2H, 3JH-H ) 7.1 Hz)
7.00 (t, 4H, 3JH-H ) 7.1 Hz)
6.62 (t, 2H, 3JH-H ) 6.9 Hz)
6.98 (t, 4H, 3JH-H ) 6.9 Hz)
Zr(OCHPh2)Cl2[N(SiMe2CH2PPri2)2], 7. The complex was pre-
pared by a procedure similar to the one described above for 6, using
ZrCl3[N(SiMe2CH2PPri2)2] (1.25 g, 2.12 mmol) and Ph2CHONa‚THF
(589 mg, 2.12 mmol). The product was crystallized from a solvent
mixture containing Et2O and pentane (1.23 g, 78%). 1H NMR (δ,
200.132 MHz, C6D6): 0.50 (s, 12H, Si(CH3)2); 0.94 (m, 28H, SiCH2P
and increased in intensity with decreasing temperature (down to -93
°C). Anal. Calcd for a sample containing only the major isomer,
C26H53ON2P2Si2Zr: C, 50.44; H, 8.63; N, 4.53. Found: C, 50.70; H,
8.87; N, 4.33. Anal. Calcd for a sample containing a mixture of major
isomer and minor isomer (major:minor ) 2:1), C26H53ON2P2Si2Zr: C,
50.44; H, 8.63; N, 4.53. Found: C, 50.24; H, 8.71; N, 4.29. Resonance
Raman (cm-1), solid (14N2): 258m, 314vs, 350w, 595m, 732s, 751s,
989w, 1046m. MS (EI), m/z: 1236, 1193, 1107, 1063, 1007, 975, 695,
350, 262.
3
and P[CH(CH3)2]2); 1.73 (sept, 4H, P[CH(CH3)2]2, JH-H ) 6.8 Hz);
3
6.82 (s, 1H, CHPh); 7.00 (2H, t, p-Ph, JH-H ) 7.6 Hz); 7.16 (4H, t,
3
m-Ph, 3JH-H ) 7.6 Hz); 7.71 (4H, d, o-Ph, JH-H ) 7.6 Hz). 31P{1H}
{[(Pri2PCH2SiMe2)2N]Zr(O-2,6-Me2C6H3}2(µ-η2:η2-15N2), 9-15N2.
The nitrogen-15 analogue was prepared by a procedure similar to that
for 9, but by introducing 15N2 gas into the flask containing the degassed
reaction mixture. Workup was carried out under unlabeled N2. 15N-
{1H} NMR (δ, 30.406 MHz, C7D8): minor isomer, 9a, 342.91 (s); major
isomer, 9, 339.06 (s). Resonance Raman (cm-1), solid (15N2): 277m,
309s, 350w, 576m, 596w, 725s, 750w, 1006vw, 1031w. MS (EI),
m/z: 1238, 1195, 1111, 977, 696, 350, 262.
NMR (δ, 81.015 MHz, C6D6): 13.53 (s). Anal. Calcd for C31H55-
Cl2ONP2Si2Zr: C, 50.45; H, 7.51; N, 1.90. Found: C, 51.25; H, 7.67;
N, 1.76.
Zr(NPh2)Cl2[N(SiMe2CH2PPri2)2], 8. To a solution of ZrCl3-
[N(SiMe2CH2PPri2)2] (1.25 g, 2.12 mmol) in THF (60 mL) was added
a solution of NaNPh2 (199 mg, 2.12 mmol) in THF (20 mL) at RT,
and the mixture was stirred for 2 h. The solvent was stripped off under
vacuum, the residues were extracted with toluene (20 mL), and the
extracts were filtered through a layer of Celite. The product was
crystallized from a solvent mixture containing toluene and hexanes (1.15
g, 75%). 1H NMR (δ, 400 MHz, C6D6): 0.47 (s, 12H, Si(CH3)2); 1.16
and 1.10 (m, 28H, SiCH2P and P[CH(CH3)2]2); 2.12 (sept, 4H,
{[(Pri2PCH2SiMe2)2N]Zr(OBut)}2(µ-η2:η2-N2), 10. A solution of
crude Zr(OBut)Cl2[N(SiMe2CH2PPri2)2] (approximately 1.05 g, 1.48
mmol) was dissolved in toluene (100 mL), and the solution was
transferred into a thick-walled reaction flask (300 mL) containing Na/
Hg (80 g of 0.30% amalgam, 10.4 mmol of Na). The flask was then
cooled to -196 °C, filled with 1 atm of N2, sealed, and allowed to
warm slowly to RT with stirring. The colorless solution slowly took
on the deep purple color of the product. The reaction mixture was
stirred for 5 d, and the solution was decanted and filtered through a
layer of Celite. Stripping off the solvent from the filtrate gave a dark
purple oil. Attempts to crystallize the product were not successful. 1H
NMR (δ, 300 MHz, C6D6): 0.46 and 0.42 (s, 12H, Si(CH3)2); 1.24 (br
m, 28H, SiCH2P and P[CH(CH3)2]2); 1.42 (s, 9H, OC(CH3)3); 1.97 (br
3
3
P[CH(CH3)2]2, JH-H ) 4.0 Hz); 6.96 (2H, t, p-Ph, JH-H ) 8.0 Hz);
3
3
7.23 (4H, t, m-Ph, JH-H ) 8.0 Hz); 7.30 (4H, d, o-Ph, JH-H ) 8.0
Hz). 31P{1H} NMR (δ, 81.015 MHz, C6D6): 15.58 (s). 31P{1H} NMR
in a solvent mixture containing THF and C6D6 (δ): 2.70 (br); -1.20
(br). 13C{1H} NMR (δ, 50.323 MHz, C6D6): 5.04 (s, SiCMe2); 9.69
(s, CH2Si); 18.85 and 19.59 (s, CH(CH3)2); 24.28 (t, CH(CH3)2, 1JP-C
) 5.6 Hz); 123.41 (s, Ph); 126.98 (s, Ph); 128.26 (s, Ph). Anal. Calcd
for C30H54Cl2N2P2Si2Zr: C, 49.83; H, 7.53; N, 3.88. Found: C, 50.09;
H, 7.56; N, 4.00.
3
{[(Pri2PCH2SiMe2)2N]Zr(O-2,6-Me2C6H3)}2(µ-η2:η2-N2), 9.
A
sept, 2H, P[CH(CH3)2]2, JH-H ) 7.2 Hz); 2.25 (br sept, 2H,
3
P[CH(CH3)2]2, JH-H ) 6.6 Hz). 31P{1H} NMR (δ, 81.015 MHz,
solution of Zr(O-2,6-Me2-C6H3)Cl2[N(SiMe2CH2PPri2)2] (1.05 g, 1.48
mmol) in toluene (100 mL) was transferred into a thick-walled reaction
flask (300 mL) containing Na/Hg (80 g of 0.17% amalgam, 5.74 mmol
of Na). The flask was then cooled to -196 °C, filled with 1 atm of
N2, sealed, and allowed to warm slowly to RT with stirring. The
colorless solution slowly took on the deep blue color of the product.
The reaction mixture was stirred for 5 d; the solution was then decanted
from the amalgam and filtered through a layer of Celite. The amalgam-
containing residue was extracted with several 50 mL portions (ap-
proximately 400 mL) of toluene, until the extracts showed no blue color.
The filtrate and the extracts were combined, and stripping off the solvent
gave a deep blue solid, which was washed with hexanes (2 × 25 mL).
Pure product was obtained by slow evaporation of a toluene solution
of the crude product at RT (0.36 g, 40%). 1H{31P} NMR (δ, 500 MHz,
C7D8): see Table 1. 31P{1H} NMR (δ, 121.421 MHz, C7D8), at 20
°C: major isomer 8.69 (s); minor isomer 8.85 (s) and 11.26 (s).
NOEDIFF experiments (δ, 400 MHz, C7D8): irradiating the resonances
at 7.00 or 6.98 ppm showed enhancements at 2.34 and 2.32 ppm.
Variable-temperature 31P{1H} NMR (δ, 121.42 MHz, C7D8): upon
cooling of a sample of pure major isomer, the resonance at 8.69 ppm
broadened and below -40 °C began to show shoulders at 7.60 and
9.10 ppm. Below -78 °C, a broad peak began to appear at 4.00 ppm
C6D6): 8.16 (s). 15N{1H} NMR (δ, 30.406 MHz, C7D8): 346.41 (s);
334.35 (s); 319.64 (s); 254.87 (s); 248.86 (s).
X-ray Crystallographic Analyses of Zr(O-2,6-Me2C6H3)Cl2-
[N(SiMe2CH2PPri2)2], 5, and {[(Pri2PCH2SiMe2)2N]Zr(O-2,6-Me2-
C6H3)}2(µ-η2:η2-N2), 9. Crystallographic data appear in Table 2. The
final unit-cell parameters were obtained by least-squares calculations
on the setting angles for 25 reflections with 2θ ) 29.6-36.1° for 5
and 46.4-72.7° for 9. The intensities of three standard reflections,
measured every 200 reflections throughout the data collections, decayed
linearly for both 5 (1.3%) and 9 (2.0%). The data were processed23
and corrected for Lorentz and polarization effects, decay, and absorption
(empirical; based on azimuthal scans).
The structures were solved by the Patterson method. The dinuclear
molecule 9 lies on a center of symmetry in the unit cell. The non-
hydrogen atoms were refined with anisotropic thermal parameters.
Hydrogen atoms were fixed in calculated positions with C-H ) 0.98
Å and BH ) 1.2Bbonded atom
. Secondary extinction corrections were
applied (Zachariasen type 1 isotropic), the final values of the extinction
(23) Crystal Structure Analysis Package; Version 1.7; Molecular Structure
Corp.: The Woodlands, TX, 1995.

A Zirconium Dinitrogen Complex
Inorganic Chemistry, Vol. 37, No. 1, 1998 115
Table 2. Crystallographic Data
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
{[(Pri2PCH2SiMe2)2N]Zr(O-2,6-Me2C6H3)}2(µ-η2:η2-N2), 9a
5
9
Zr(1)-P(1)
Zr(1)-O(1)
Zr(1)-N(2)
P(1)-C(1)
P(1)-C(8)
P(2)-C(13)
Si(1)-N(1)
Si(1)-C(3)
Si(2)-N(1)
Si(2)-C(5)
O(1)-C(19)
2.818(1)
2.020(3)
2.034(4)
1.824(5)
1.851(6)
1.844(5)
1.718(4)
1.878(6)
1.720(4)
1.861(6)
1.331(6)
Zr(1)-P(2)
Zr(1)-N(1)
Zr(1)-N(2)′
P(1)-C(7)
P(2)-C(2)
P(2)-C(14)
Si(1)-C(1)
Si(1)-C(4)
Si(2)-C(2)
Si(2)-C(6)
N(2)-N(2)′
2.846(1)
2.211(3)
2.082(4)
1.848(6)
1.813(5)
1.852(6)
1.890(5)
1.870(6)
1.887(5)
1.881(5)
1.528(7)
formula
fw
crystal system
space group
a, Å
b, Å
c, Å
C26H53Cl2NOP2Si2Zr
675.96
C52H106N4O2P4Si4Zr2
1238.11
orthorhombic
Pbcn (No. 60)
14.3839(14)
17.7539(14)
25.9967(10)
90
6638.8(7)
4
1.239
21
Cu
monoclinic
P21/c (No. 14)
13.442(2)
16.777(2)
16.456(2)
108.564(9)
3518.1(7)
4
1.276
21
Mo
0.710 69
0.64
â, deg
V, Å3
Z
F
calcd, g/cm3
T, °C
radiation
λ, Å
P(1)-Zr(1)-P(2)
P(1)-Zr(1)-N(1)
P(1)-Zr(1)-N(2)′
P(2)-Zr(1)-N(1)
P(2)-Zr(1)-N(2)′
O(1)-Zr(1)-N(2)
N(1)-Zr(1)-N(2)
N(2)-Zr(1)-N(2)′
Zr(1)-P(1)-C(7)
C(1)-P(1)-C(7)
C(7)-P(1)-C(8)
Zr(1)-P(2)-C(13) 115.8(2)
C(2)-P(2)-C(13) 102.3(2)
C(13)-P(2)-C(14) 109.9(3)
N(1)-Si(1)-C(3)
C(1)-Si(1)-C(3)
C(3)-Si(1)-C(4)
N(1)-Si(2)-C(5)
C(2)-Si(2)-C(5)
C(5)-Si(2)-C(6)
145.54(4)
79.4(1)
85.10(10) P(2)-Zr(1)-O(1)
74.5(1)
P(1)-Zr(1)-O(1)
P(1)-Zr(1)-N(2)
87.6(1)
128.10(10)
88.1(1)
82.89(10)
123.0(1)
1.541 78
4.55
0.037
µ, cm-1
Ra
P(2)-Zr(1)-N(2)
0.035
0.031
126.19(10) O(1)-Zr(1)-N(1)
b
Rw
0.034
119.8(1)
111.4(1)
43.6(2)
115.1(2)
106.3(3)
107.6(3)
O(1)-Zr(1)-N(2)′ 119.1(1)
N(1)-Zr(1)-N(2)′ 114.7(1)
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
2
Zr(1)-P(1)-C(1)
Zr(1)-P(1)-C(8)
C(1)-P(1)-C(8)
Zr(1)-P(2)-C(2)
101.3(2)
120.1(2)
104.9(3)
93.3(2)
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Zr(O-2,6-Me2C6H3)Cl2[N(SiMe2CH2PPri2)2], 5
Zr(1)-Cl(1)
Zr(1)-P(1)
Zr(1)-O(1)
P(1)-C(1)
P(1)-C(8)
P(2)-C(13)
Si(1)-N(1)
Si(1)-C(3)
Si(2)-N(1)
Si(2)-C(5)
O(1)-C(19)
2.4732(9)
2.790(1)
1.969(2)
1.819(4)
1.848(3)
1.842(3)
1.733(3)
1.880(4)
1.736(3)
1.879(4)
1.362(4)
Zr(1)-Cl(2)
Zr(1)-P(2)
Zr(1)-N(1)
P(1)-C(7)
P(2)-C(2)
P(2)-C(14)
Si(1)-C(1)
Si(1)-C(4)
Si(2)-C(2)
Si(2)-C(6)
2.4450(9)
2.769(1)
2.166(2)
1.844(3)
1.821(3)
1.846(3)
1.892(4)
1.863(4)
1.894(3)
1.862(4)
Zr(1)-P(2)-C(14) 124.4(2)
C(2)-P(2)-C(14)
N(1)-Si(1)-C(1)
N(1)-Si(1)-C(4)
C(1)-Si(1)-C(4)
N(1)-Si(2)-C(2)
N(1)-Si(2)-C(6)
C(2)-Si(2)-C(6)
106.5(3)
110.3(2)
112.3(2)
105.4(2)
108.2(2)
115.4(2)
106.3(2)
115.4(2)
107.7(3)
105.1(3)
112.4(2)
107.7(2)
106.4(3)
Zr(1)-O(1)-C(19) 161.5(4)
Zr(1)-N(1)-Si(2) 116.4(2)
Zr(1)-N(2)-Zr(1)′ 130.7(2)
Zr(1)-N(1)-Si(1) 123.9(2)
Si(1)-N(1)-Si(2)
Zr(1)-N(2)-N(2)′
a Primes refer to the symmetry operation: -x, y, /2 - z.
119.7(2)
69.9(2)
Zr(1)′-N(2)-N(2)′
66.5(2)
Cl(1)-Zr(1)-Cl(2) 177.14(3) Cl(1)-Zr(1)-P(1)
91.25(3)
90.07(7)
91.34(3)
88.34(7)
155.58(3)
76.57(7)
81.34(7)
97.5(1)
1
Cl(1)-Zr(1)-P(2)
Cl(1)-Zr(1)-N(1)
Cl(2)-Zr(1)-P(2)
Cl(2)-Zr(1)-N(1)
P(1)-Zr(1)-O(1)
P(2)-Zr(1)-O(1)
O(1)-Zr(1)-N(1)
Zr(1)-P(1)-C(7)
C(1)-P(1)-C(7)
C(7)-P(1)-C(8)
Zr(1)-P(2)-C(13)
C(2)-P(2)-C(13)
79.16(3) Cl(1)-Zr(1)-O(1)
92.03(7) Cl(2)-Zr(1)-P(1)
98.91(3) Cl(2)-Zr(1)-O(1)
89.75(7) P(1)-Zr(1)-P(2)
98.77(6) P(1)-Zr(1)-N(1)
103.60(6) P(2)-Zr(1)-N(1)
174.92(9) Zr(1)-P(1)-C(1)
CH2PPri2)2] fragment. We reasoned that replacement of the
chloride by another hard, anionic ligand, such as an alkoxide
(OR) or an amide (NR2), should not drastically affect the frontier
orbitals of the fragment, and thus, a dinitrogen complex with
the N2 side-on bound should result. This premise was based
on semiempirical calculations on the fragments trans-Zr(NH2)X-
(PH3)2, which show that, for X ) halide, OH, or NH2, the two
metal-based orbitals that are available to overlap with the π*
orbitals on the N2 fragment are those that prefer the side-on
mode.20 Our initial attempts to utilize the dinitrogen complex
1 as the starting material by direct substitution of the Cl ligand
failed since addition of a variety of alkoxide or aryloxide
reagents to 1 resulted in either decomposition or no reaction.
However, as the starting material for both 1 and 2 is the Zr(IV)
complex ZrCl3[N(SiMe2CH2PPri2)2] (4), we examined func-
tionalization of this complex.
Addition of 1 equiv of sodium 2,6-dimethylphenolate or
potassium tert-butoxide to 4 resulted in the conversion to the
derivatives Zr(OR)Cl2[N(SiMe2CH2PPri2)2] (5, R ) 2,6-
Me2C6H3; 6, R ) But); sodium diphenylmethoxide, NaOCHPh2,
also produced the monoalkoxo complex 7. In the case of the
tert-butoxide complex 6, we were unable to purify it from a
number of side products, and this complicated reduction under
N2. However, we were able to prepare both the aryloxide
derivative 5 and the substituted alkoxide 7 in high purity. In
addition, the diphenylamido complex Zr(NPh2)Cl2[N(SiMe2CH2-
PPri2)2] (8) was synthesized by a similar metathesis process
using sodium diphenylamide.
125.0(1)
107.4(2)
108.1(2)
115.1(1)
104.1(2)
Zr(1)-P(1)-C(8)
C(1)-P(1)-C(8)
Zr(1)-P(2)-C(2)
110.2(1)
107.0(2)
99.5(1)
Zr(1)-P(2)-C(14) 122.9(1)
C(2)-P(2)-C(14)
N(1)-Si(1)-C(1)
N(1)-Si(1)-C(4)
C(1)-Si(1)-C(4)
N(1)-Si(2)-C(2)
N(1)-Si(2)-C(6)
C(2)-Si(2)-C(6)
105.4(2)
107.8(1)
114.2(1)
107.2(2)
109.4(1)
112.6(1)
106.5(2)
C(13)-P(2)-C(14) 107.3(2)
N(1)-Si(1)-C(3)
C(1)-Si(1)-C(3)
C(3)-Si(1)-C(4)
N(1)-Si(2)-C(5)
C(2)-Si(2)-C(5)
C(5)-Si(2)-C(6)
Zr(1)-N(1)-Si(1)
Si(1)-N(1)-Si(2)
113.8(2)
106.6(2)
106.7(2)
113.3(1)
107.6(2)
107.1(2)
120.9(1)
117.9(1)
Zr(1)-O(1)-C(19) 170.5(2)
Zr(1)-N(1)-Si(2) 121.2(1)
coefficients being 2.05(5) × 10-7 for 5 and 9.2(5) × 10-8 for 9.
Neutral-atom scattering factors and anomalous dispersion corrections
were taken from ref 24.
Selected bond lengths and bond angles for the 5 and 9 appear in
Tables 3 and 4, respectively. Tables of final atomic coordinates and
equivalent isotropic thermal parameters, anisotropic thermal parameters,
bond lengths and angles, torsion angles, intermolecular contacts, and
least-squares planes are included as Supporting Information.
Results and Discussion
Synthesis and Structure of Zr(IV) Precursors. Each end
of the side-on dinitrogen complex 1 contains a ZrCl[N(SiMe2-
(24) (a) International Tables for X-Ray Crystallography; Kynoch Press:
Birmingham, U.K., 1974; Vol. IV, pp 99-102. (b) International Tables
for Crystallography; Kluwer Academic Publishers: Boston, MA, 1992;
Vol. C, pp 200-206.
The stereochemistry of these monosubstituted complexes is
assumed to be distorted octahedral with the alkoxide, aryloxide,
or amide trans to the amide of the ancillary tridentate ligand.

116 Inorganic Chemistry, Vol. 37, No. 1, 1998
Cohen et al.
All of the complexes 5-8 show very similar NMR spectral
characteristics consistent with C2V molecular symmetry; in
particular, the silylmethyl protons (SiMe2) appear as a singlet
Figure 1. ORTEP drawing showing the molecular structure and
numbering scheme for Zr(O-2,6-Me2C6H3)Cl2[N(SiMe2CH2PPri2)2] (5).
Ellipsoids are drawn at the 33% probability level.
1
in the H NMR spectrum and the methylene protons of the
backbone (PCH2Si) give rise to a simple doublet due to coupling
to phosphorus-31. This stereochemistry was confirmed for the
aryloxide complex 5 in the solid state.
routinely passed the N2 through a MnO/molecular sieve tower
to remove trace O2 and H2O, the crude reaction mixture obtained
from the reduction of 5 under purified tank N2 possessed a
number of impurities as evidenced from 31P{1H} NMR spectra.
Most of the impurities were hexanes-soluble and could be
removed by washing the crude precipitate with hexanes;
recrystallization of the remaining material from toluene gave
the pure N2 complex 9 having only a singlet resonance at 8.69
ppm in the 31P{1H} NMR spectrum. In contrast, when the
reduction was performed under 15N-labeled dinitrogen, 15N2, not
only were there fewer impurities present but, in addition, two
dinitrogen-containing complexes could be isolated, both having
the same (but isotopomeric) molecular formula as 9, in other
words, {[(Pri2PCH2SiMe2)2N]Zr(O-2,6-Me2C6H3)}2(µ-η2:η2-
15N2) (9-15N2); presumably, the labeled dinitrogen is of better
quality than the unlabeled N2. The 31P{1H} NMR spectrum of
the mixture showed three singlets at 8.69, 8.85, and 11.26 ppm
in the approximate ratio 4:1:1; assuming that the two new
singlets at 8.85 and 11.26 ppm are due to a single isomeric
compound, labeled 9a, then the isomers formed are in a 2:1
ratio with the major isomer, labeled 9, giving rise to the singlet
at 8.69 ppm. The 1H NMR spectral resonances of the mixture
are detailed in Table 1. One possible rationalization of the NMR
spectral data is that the major isomer 9 has both ends of the
dinuclear unit equivalent, while the minor isomer 9a has
inequivalent Zr(O-2,6-C6H3)[N(SiMe2CH2PPri2)2] fragments at-
tached to the bridging N2 moiety. Since the minor isomer 9a
is more soluble in hexanes, separation of the two isomers can
be achieved by washing the solid mixture of 9 and 9a with
hexanes; recrystallization of the remaining material from toluene
gives pure 9. Attempts to obtain pure 9a have so far failed.
Up to this point, we have not presented any data to show
that the dinitrogen unit in either 9 or 9a is side-on bound. As
we discussed earlier, differentiating between side-on and end-
on coordination of N2 in a dinuclear complex is difficult. NMR
spectral correlations, even 15N NMR chemical shifts, are
ambiguous. However, we have shown that resonance Raman
spectroscopy has proven diagnostic of the bonding mode and
can be correlated to single-crystal X-ray data.21
The molecular structure and numbering scheme are shown
in Figure 1, and selected bond lengths and angles are given in
Table 3. The distortion away from octahedral is most evident
in the P(1)-Zr(1)-P(2) angle of 155.58(3)° whereas the other
transoid angles are closer to 180°: O(1)-Zr(1)-N(1) is
174.92(9)° and Cl(1)-Zr-Cl(2) is 177.14(3)°. In addition, the
Zr(1)-O(1)-C(19) bond angle of 170.5(2)° indicates that there
is some multiple-bond character in the Zr-O bond possibly due
to π-donation from the oxygen to Zr. The Zr(1)-O(1) bond
length of 1.969(2) Å is short as compared to those of other
systems.25 These two interrelated structural features are indica-
tors of metal-oxygen dπ-pπ interactions where larger bond
angles and shorter bond lengths are correlated with π-electron
donation from the oxygen.25 By comparison, parameters
associated with the zirconium-oxygen bond for {[(But)3-
CO]2ZrCl3‚Li(OEt2)2}25 are 1.859 Å and 169°; for [Cp2Zr-
(NMe2)(O-2,6-But2C6H3)],26 they are 2.056(1) Å and 142.7(7),°
and for [(Cp2ZrMe)2O],27 1.945 Å and 174.1° are found. Other
bond lengths are typical of those previously found for Zr(IV)
complexes with this particular ligand system.20
Reduction under Dinitrogen. The reaction of aryloxide 5
with >2 equiv of Na/Hg in toluene under 1-4 atm of N2
produced the dinuclear dinitrogen complex {[(Pri2PCH2SiMe2)2N]-
Zr(O-2,6-Me2C6H3)}2(µ-η2:η2-N2) (9) in 40% yield (see below).
Reduction of the impure tert-butoxide derivative 6 was also
examined; although deep purple solutions were obtained that
are diagnostic of dinuclear dinitrogen complexes (e.g., both 1
and 9 are dark blue both in solution and in the solid state as is
3, and 2 is deep brown), crystalline materials could not be
isolated and the solution NMR spectra indicated multiple
products. Attempts to reduce the diphenylmethoxide complex
7 and the diphenylamide derivative 8 did not lead to any
tractable materials, and no evidence of reduction by the
formation of colored solutions was obtained. (See Scheme 1.)
The formation of the aryloxide dinitrogen complex 9 was
sensitive to the purity of the N2 atmosphere. Although we
(25) Lubben, T. V.; Wolczanski, P. T.; vanDuyne, G. D. Organometallics
1984, 3, 977.
(26) Cardin, D. J.; Lappert, M. F.; Raston, C. L. Chemistry of Organo-
Zirconium and Hafnium Compounds, 1st ed.; Ellis Horwood Lim-
ited: Toronto, 1986.
(27) Hunter, W. E.; Hrncir, D. C.; Bynum, R. V.; Penttila, R. A.; Atwood,
J. L. Organometallics 1983, 2, 750.
The single-crystal X-ray structure of the major isomer 9 is
shown in Figure 2, and selected bond distances and bond angles
are given in Table 4. The solid state structure unequivocally
shows that the dinitrogen is bound in a side-on fashion bridging
the two zirconium centers. The observed nitrogen-nitrogen

A Zirconium Dinitrogen Complex
Inorganic Chemistry, Vol. 37, No. 1, 1998 117
Scheme 1
zirconium that contain this PNP ancillary ligand system. The
bond lengths from the zirconium to each of the bridging
nitrogens, Zr-N(2), are 2.034(4) and 2.082(4) Å, shorter than
the zirconium-amide bond length, Zr-N(1) ) 2.211(3) Å, of
the ancillary tridentate ligand; similar differences in Zr-N bond
distances were observed for the chloride complex 1. Other
zirconium-nitrogen bond distances in the literature range from
1.826(4) Å for a zirconium-nitrogen double bond in Cp2Zr )
NBut(THF)28 to 2.443(1) Å in Schiff base chelate derivatives.29
The zirconium-oxygen bond separation, Zr-O(1), is
2.020(3) Å and the bond angle defined by the atoms Zr, O(1),
and C(19) is 161.5(4)° (Table 4). In the solid state structure of
8, the aryloxide ligand is oriented such that the methyl
substituents are directed away from the isopropyl substituents
of the phosphine donors.
The structure of the major isomer 9 having each aryloxide
ligand on the same, closed side of the Zr2N2 hinge suggests
that the minor isomer, 9a, could have the aryloxide units on
opposite sides of the hinged Zr2N2 core. Assuming that the
Zr2N2 core remains hinged, the major isomer 9 has C2V
symmetry while the minor isomer has Cs symmetry. Such a
proposal is consistent with the NMR spectroscopic parameters
of both the major and minor isomers (Table 1); in particular,
the dinuclear unit 9a with Cs symmetry would have inequivalent
ends and, as such, would give rise to more complicated 1H NMR
spectral data and should also show inequivalent phosphorus
nuclei in the 31P{1H} NMR spectrum, exactly in agreement with
the data.
Figure 2. ORTEP drawing showing the molecular structure and
numbering scheme for {[(Pri2PCH2SiMe2)2N]Zr(O-2,6-Me2C6H3)}2(µ-
η2:η2-N2) (9). Ellipsoids are drawn at the 33% probability level.
bond length of 1.528(7) Å (Table 4) is essentially identical to
that (1.548(7) Å) previously reported for the side-on derivative
1 and is significantly longer than the bridging end-on complexes
where the nitrogen-nitrogen bond lengths range from 1.12 to
1.3 Å. A compilation of N-N bond lengths (Table 5) provides
a comparison with other dinitrogen complexes and also to
selected other molecules of interest. The N-N bond lengths
in complexes 1 and 9 are not only longer than that of free
hydrazine but also longer than that found for the coordinated
hydrazine ligand (N2H4) or hydrazido ligand (NHNH)2- (Table
5). Another important structural feature is that the bridging N2
unit is not planar as found for 1 but exhibits a bent or hinged
geometry. The hinge angle, defined by the angle between the
two ZrN2 planes, is 156.2°. It can also be seen that the aryloxide
substituents lie on the same closed side of the Zr2N2 hinge; this
is in contrast to that found in 1 where the chloride ligands on
Zr are located on opposite sides of the planar Zr2N2 core. As
will be discussed below, the hinged structural motif can be used
to rationalize the observation of structural isomers in the crude
reaction mixture of 9.
To further characterize these complexes, resonance Raman
(RR) data were collected for 9. With excitation at 647.1 nm,
two strong bands are observed at 751 and 732 cm-1 (Figure
3A). The 751 cm-1 peak shifts to 725 cm-1, overlapping with
the 732 cm-1 band in the spectrum of the 15N2 isotopomer and
(28) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc.
1988, 110, 8729.
(29) Archer, R. D.; Day, R. O.; Illingsworth, M. L. Inorg. Chem. 1976,
18, 2908.
(30) Wilkinson, P. G.; Hounk, N. B. J. Chem. Phys. 1956, 24, 528.
(31) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
(32) Sutton, L. E. Tables of Interatomic Distances and Configurations in
Molecules and Ions; Chemical Society: London, 1958; Vol. 11.
(33) Fryzuk, M. D.; Haddad, T. S.; Rettig, S. J. J. Am. Chem. Soc. 1990,
112, 8185.
(34) Turner, H. W.; Fellmann, J. D.; Rocklage, S. M.; Schrock, R. R.;
Churchill, M. R.; Wasserman, H. J. J. Am. Chem. Soc. 1980, 102,
7810.
(35) Blum, L.; Williams, I. D.; Schrock, R. R. J. Am. Chem. Soc. 1984,
106, 8316.
(36) Duchateau, R.; Gambarotta, S.; Beydoun, N.; Bensimon, C. J. Am.
Chem. Soc. 1991, 113, 8986.
The bond distances associated with the phosphine donors and
zirconium are comparable to those in other complexes of

118 Inorganic Chemistry, Vol. 37, No. 1, 1998
Cohen et al.
Table 5. Compilation of Nitrogen-Nitrogen Bond Lengths for Selected Compounds
compound
bond length, Å
ref
N2
1.0975(2)
1.255
1.46
1.088(12)
1.548(7)
1.301(3)
1.528(7)
1.525
1.43(1)
1.298(12)
30
31
32
15
20, 33
20
this work
16
19
PhNdNPh
H2NNH2
[(η5-C5Me5)2Sm[2(µ-η2:η2-N2)
{[(Pri2PCH2SiMe2)2N]ZrCl}2(µ-η2:η2-N2), 1
{[(Pri2PCH2SiMe2)2N]Zr(η5-C5H5)}2(µ-N2), 2
{[(Pri2PCH2SiMe2)2N]Zr(O-2,6-Me2C6H3)}2(µ-η2:η2-N2), 9
[(OEPG)Li(THF)2]Sm(µ-η2:η2-N2)Li4
{[PhP(CH2SiMe2NSiMe2CH2)2PPh]Zr}2(µ-η2:η2-N2), 3
{(Me3P)2(Me3CCH2)TadCHCMe3}2(µ-N2)
[W(NPh)Me3]2(µ-η1:η1-NH2NH2)(µ-η2:η2-NHNH)
µ-η1:η1-NH2NH2
34
35
1.434(14)
1.391(15)
1.06
µ-η2:η2-NHNH
[{Li(THF)3}2(µ-η2:η2-N2)]+[{(η5-C5H5)2Zr}2(µ-PPh)2]-
[{[(Me3Si)2N]2Ti}2(µ-η2:η2-N2)2]-
13
36
1.379
Figure 3. Resonance Raman spectra of 9: (A) 14N2 isotopomer; (B)
15N2 isotopomer. Spectra were collected with 647.1 nm excitation (60
mW) at ≈90 K using a scan rate of 1 cm-1/s for nine scans. Each
spectrum was 13-point-smoothed and baseline-corrected.
Other peaks in the RR spectrum of 9 also show isotope shifts.
To illustrate, there is a peak at 595 cm-1 in the spectrum of the
14N2 isotopomer (Figure 3A) that forms a doublet in the
spectrum of the 15N2 isotopomer at 596 and 576 cm-1 (Figure
3B). Again, the small peak at 596 cm-1 in Figure 3B probably
represents some residual 14N2 in the 15N2 isotopomer. The shift
from 595 to 576 cm-1 is within 2 cm-1 of the shift for a Zr-N
stretch predicted by assuming a pure diatomic oscillator. It is
interesting to note that no isotope-sensitive stretches assignable
to Zr-N modes were observed for 1, possibly a reflection of
the difference in the local symmetry between 1 and 9.
Alternatively, these isotope-sensitive stretches may be due to a
Zr2N2 cage mode similar to that proposed for the Cu2O2 dimer
in the µ-η2:η2-O2 complex of oxyhemocyanin.37
causing its intensity to increase sharply relative to the 751 cm-1
band (Figure 3B). This shift from 751 to 725 cm-1 is within 1
cm-1 of the predicted isotope shift for a pure N-N diatomic
oscillator. Consequently, we assign the peak at 751 cm-1 to
the symmetric ν(N-N) vibrational mode. This frequency is
consistent with the long N-N bond distance and the supports
the side-on-bonded orientation of the N2 ligand as found in the
crystal structure of 9. The small residual intensity at 750 cm-1
in the 15N2 spectrum is probably due to some 14N2 contaminant
in the 15N2 isotopomer. The assignment of the 732 cm-1 peak
in Figure 3A remains unclear. However, its isotope insensitivity
suggests that it may be a ligand mode not associated with the
Zr2N2 moiety. The weaker bands at 986 and 1046 cm-1 may
be assigned as combinations between the strong 732 cm-1 band
and the two isotope-sensitive, low-frequency modes at 258 and
314 cm-1, respectively. Similar combination bands are observed
for the 15N2 isotopomer at 1006 (732 + 277) and 1031 (732 +
309) cm-1. Similar RR features were observed in the RR
spectrum of {[(Pri2PCH2SiMe2)2N]ZrCl}2(µ-η2:η2-N2) (1), which
has a chloride in place of the aryloxide ligand of 9.21
Conclusions
In this work we have presented another example of the side-
on mode of bonding of a dinitrogen fragment in a dinuclear
(37) Ling, J.; Nestor, L. P.; Czernuszewicz, R. S.; Spiro, T. G.; Fraczk-
iewicz, R.; Loehr, T. M.; Sanders-Loehr, J. J. Am. Chem. Soc. 1994,
116, 7682.

A Zirconium Dinitrogen Complex
Inorganic Chemistry, Vol. 37, No. 1, 1998 119
metal complex. The use of Zr as the metal center seems to be
important, as does the choice of the ancillary ligand. For
example, it would appear that replacement of the anionic
chloride ligand of 1 by an aryloxide donor does not dramatically
affect the frontier orbitals as the side-on mode is observed and,
as well, a very long N-N bond length of 1.528(7) Å is found
in 9. However, other attempts to replace the chloride with amide
or alkoxide ligands have so far failed to produce isolable N2
complexes, presumably due to synthetic difficulties.
Acknowledgment. Financial support was generously pro-
vided by the NSERC of Canada (M.D.F.) and NIH Grant GM
18865 (T.M.L.).
Supporting Information Available: X-ray crystallographic files,
in CIF format, for the structure determinations of 5 and 9 are availasble
on the Internet only. Access information is given on any current
masthead page.
IC9709786