Reactivity of low-valent iridium, rhodium, and platinum complexes with Di- and tetrasubstituted hydrazines

Article abstract of DOI:10.1021/om701192s

Three complexes, IrCl(PEt₃)₃, Rh(NNN)Cl (NNN = 2,6-(CyN=CH)₂C₅H₃N), and (NN)PtMe₂ [NN = 1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy)], have been investigated for N-N oxidative addition reactivity with di- and tetrasubstituted hydrazines. The reaction of IrCl(PEt₃)₃with 1,2-diphenylhydrazine (PhNHNHPh) forms the cyclometalated azobenzene complex (Et₃P)₂Cl(H)Ir(C₆H₄N=NPh) (1) and the 2:1 complex [(Et₃P)₂Cl(H)Ir]₂(μ-C ₆H₄N=NC₆H₄) (2). The Rh(NNN)Cl pincer complex catalyzes the disproportionation of PhNHNHPh to azobenzene and aniline with no change in the Rh complex. The reaction of (bpy)PtMe₂ with the hydrazine AcNHNHC(O)CH₂CH₂CH=CH₂ (3) yields the platinum metallacycle (bpy)Pt(η²-AcN-NC(O)(CH ₂)₂CHCH₂ (4). Although IrCl(PEt ₃)₃, Rh(NNN)Cl, and (NN)PtMe₂ all undergo facile oxidative addition of other X-Y bonds, such reactivity is not observed for hydrazines; in the Ir and Pt systems there is a clear preference for cleavage of the stronger N-H bond over the N-N bond. The tetrasubstituted hydrazines tetraphenylhydrazine, 1,2-diphenyl-1,2-dimethylhydrazine, bisuccinimide, biphthalimide, and N-dimethylamino phthalimide do not react with IrCl(PEt₃)₃, Rh(NNN)Cl, and (NN)PtMe₂. This lack of reactivity appears to be due to kinetic rather than thermochemical factors.

Full text of DOI:10.1021/om701192s

2238
Organometallics 2008, 27, 2238–2245
Reactivity of Low-Valent Iridium, Rhodium, and Platinum
Complexes with Di- and Tetrasubstituted Hydrazines
Jessica M. Hoover, John Freudenthal,† Forrest E. Michael,* and James M. Mayer*
Department of Chemistry, Campus Box 351700, UniVersity of Washington, Seattle,
Washington 98195-1700
ReceiVed NoVember 27, 2007
Three complexes, IrCl(PEt3)3, Rh(NNN)Cl (NNN ) 2,6-(CyNdCH)2C5H3N), and (NN)PtMe2 [NN )
1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy)], have been investigated for N-N oxidative addition
reactivity with di- and tetrasubstituted hydrazines. The reaction of IrCl(PEt3)3with 1,2-diphenylhydrazine
(PhNHNHPh) forms the cyclometalated azobenzene complex (Et3P)2Cl(H)Ir(C6H4NdNPh) (1) and the
2:1 complex [(Et3P)2Cl(H)Ir]2(µ-C6H4NdNC6H4) (2). The Rh(NNN)Cl pincer complex catalyzes the
disproportionation of PhNHNHPh to azobenzene and aniline with no change in the Rh complex. The reaction
of (bpy)PtMe2 with the hydrazine AcNHNHC(O)CH2CH2CHdCH2 (3) yields the platinum metallacycle
(bpy)Pt(η2-AcN-NC(O)(CH2)2CHCH2 (4). Although IrCl(PEt3)3, Rh(NNN)Cl, and (NN)PtMe2 all undergo
facile oxidative addition of other X-Y bonds, such reactivity is not observed for hydrazines; in the Ir
and Pt systems there is a clear preference for cleavage of the stronger N-H bond over the N-N bond.
The tetrasubstituted hydrazines tetraphenylhydrazine, 1,2-diphenyl-1,2-dimethylhydrazine, bisuccinimide,
biphthalimide, and N-dimethylamino phthalimide do not react with IrCl(PEt3)3, Rh(NNN)Cl, and
(NN)PtMe2. This lack of reactivity appears to be due to kinetic rather than thermochemical factors.
metal systems,10 although the corresponding reductive elimi-
Introduction
nation reactions can be facile.11 While oxidative addition
Oxidative addition reactions are important routes to new
metal–ligand σ bonds and are fundamental steps in metal-
catalyzed reactions. Early studies of this process focused on
the addition of H2 to metal complexes to give new dihydride
species,1 followed by an interest in C-H,2 C-X,3 C-O,4 and
C-S5 bond activations. There have also been reports of the
oxidative addition of S-S,6 O-O,7 and even C-C8bonds to
yield stable high oxidation state thiolate, alkoxide, aryl, or
alkyl complexes. C-N oxidative addition reactions have been
observed primarily for strained substrates9 or low-valent early
processes are common to a variety of bonds, there is little
evidence for the simple oxidative addition of the N-N bond
of hydrazines.
Hydrazine reactivity with metal complexes has been studied
primarily in the context of the cleavage of N-N bonds by
nitrogenase enzymes.12 Our interest in hydrazine oxidative
addition derives from its possible application in catalytic olefin
diamination. Diamination is an oxidative process that forms two
C-N bonds, and there have been recent reports of catalytic
diaminations using amines and added oxidants.13 Diamination
using a hydrazine is directly analogous to the hydroamination
of olefins with amines, allowing the hydrazine to act as its own
oxidant. Such a process could occur by oxidative addition of a
hydrazine to a metal center to make an oxidizing bis(amido)
complex followed by addition of the amido ligands to an alkene
with a two-electron reduction of the metal center. A related
reaction is the addition of a diaziridinone to dienes catalyzed
* Corresponding authors. E-mail: michael@chem.washington.edu;
mayer@chem.washington.edu.
† University of Washington X-ray Crystallography Facility.
(1) (a) Vaska, L.; Rhodes, R. E. J. Am. Chem. Soc. 1965, 87, 4970–
4971. (b) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem.
Soc. A 1966, 1711–1732. (c) For a review on hydride complexes: Kaesz,
H. D.; Saillant, R. B Chem. ReV. 1972, 72, 231–281.
(2) (a) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879–2932.
(b) Crabtree, R. H. J. Organomet. Chem. 2004, 689, 4083–4091.
(3) (a) Grushin, V. V.; Alper, H. Chem. ReV. 1994, 94, 1047–1062. (b)
Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. ReV. 1994, 94,
373–431.
(4) (a) Ishizu, J.; Yamamoto, T.; Yamamoto, A. Chem. Lett. 1976, 1091–
1094. (b) Yamamoto, T.; Ishizu, J.; Kohara, T.; Komiya, S.; Yamamoto,
A. J. Am. Chem. Soc. 1980, 102, 3758–3764. (c) Yamamoto, T.; Ishizu, J.;
Yamamoto, A. J. Am. Chem. Soc. 1981, 103, 6863–6869.
(5) (a) Gosselink, J. W.; van Koten, G.; Spek, A. L.; Duisenberg, A. J. M.
Inorg. Chem. 1981, 20, 877–884. (b) Osakada, K.; Chiba, T.; Nakamura,
Y.; Yamamoto, T.; Yamamoto, A. J. Chem. Soc., Chem. Commun. 1986,
1589–1591.
(10) (a) Bonanno, J. B.; Henry, T. P.; Neithamer, D. R.; Wolczanski,
P. T.; Lobvsky, E. B. J. Am. Chem. Soc. 1996, 118, 5132–5133. (b)
Tayebani, M.; Gambarotta, S.; Yap, G. Organometallics 1998, 17, 3639–
3641.
(11) (a) Hartwig, J. F.; Richards, S.; Barañano, D.; Paul, F. J. Am. Chem.
Soc. 1996, 118, 3626–3633. (b) Driver, M. S.; Hartwig, J. F. J. Am. Chem.
Soc. 1997, 119, 8232–8245. (c) Pawlikowski, A. V.; Getty, A. D.; Goldberg,
K. I. J. Am. Chem. Soc. 2007, 129, 10382–10393.
(6) Beletskaya, I.; Moberg, C. Chem. ReV. 1999, 99, 3435–3461.
(7) Hlavica, R. Eur. J. Biochem. 2004, 271, 4335–4360.
(8) Jun, C.-H. Chem. Soc. ReV. 2004, 33, 610–618.
(12) (a) Bernskoetter, W. H. Pool, J. A. Lobkovsky, E. Chirik, P. J.
J. Am. Chem. Soc. 2005, 127, 7901–7911. (b) Bernskoetter, W. H.
Lobkovsky, E. Chirik, P. J. J. Am. Chem. Soc. 2005, 127, 14051–14061.
(c) Zhang, Q.-F. Zheng, H. Wong, W.-Y. Wong, W.-T. Leung, W.-H. Inorg.
Chem. 2000, 39, 5255–5264. (d) Barrière, F. Coord. Chem. ReV. 2003, 236,
71–89 (e) References 15–21.
(9) (a) Lin, B. L.; Clough, C. R.; Hillhouse, G. L. J. Am. Chem. Soc.
2002, 124, 2890–2891. (b) Ozerov, O. V.; Guo, C.; Papkov, V. A.; Foxman,
B. M. J. Am. Chem. Soc. 2004, 126, 4792–4793. (c) Fan, L.; Yang, L.;
Guo, C.; Foaxman, B. M.; Ozerov, O. V. Organometallics 2004, 23, 4778–
4787. (d) Weng, W.; Guo, C.; Moura, C.; Yang, L.; Foxman, B. M.; Ozerov,
O. V. Organometallics 2005, 24, 3487–3499.
(13) (a) Bar, G. L. J.; Lloyd-Jones, G. C.; Booker-Milburn, K. I. J. Am.
Chem. Soc. 2005, 127, 7308–7309. (b) Streuff, J.; Hövelmann, C. H.; Nieger,
M.; Muñiz, K. J. Am. Chem. Soc. 2005, 127, 14586–14587.
10.1021/om701192s CCC: $40.75
2008 American Chemical Society
Publication on Web 04/16/2008

ReactiVity of Low-Valent Ir, Rh, and Pt Complexes
Organometallics, Vol. 27, No. 10, 2008 2239
by Pd0, which is proposed to proceed through an oxidative
addition pathway.14
has not been observed in typical organometallic systems that
undergo oxidative addition.
The oxidative addition of an N-N bond to a metal center to
yield a bis(amido) complex has not been reported. N-N
cleavage has been observed with sulfur- and hydride-ligated
metal clusters,15 low-valent early metal complexes,16 and Mo
and W systems developed by Schrock and co-workers.17
Reduced iron and ruthenium complexes have been reported to
cleave the N-N bond of PhNHNHPh in multistep processes to
give amido or imido dimers or oligomers, such as Cp*2Fe2(µ-
NPh)2 from Cp*Fe[N(SiMe3)2],18 Fe4(µ-NPh)4(SAr)4 from the
in situ generated Fe[N(SiMe3)2](SAr),19 and our recent report
of Cp*2Ru2(µ-NHPh)2Cl2 from [Cp*RuCl]4.20 Holland and co-
workers have described binding of N2H4 to LFe(µ-H)2BEt2
(L ) 2,4-bis(2,6-diisopropylphenylimino)pent-2-yl) to give an
η1-hydrazine complex, which upon heating cleaves the N-N
bond to form the diaminoborate complex LFe(µ-NH2)2BEt2 with
loss of H2.21 The cleavage of azines by a (PCP)Rh(N2) species
(PCP ) 2,6-(CH2PR3)2C6H3) to give the corresponding imine
and nitrile complexes has been reported.22 Imido complexes are
formed by N-N cleavage in some systems, such as the cleavage
of the NdN bond of azobenzene by a tungsten(II) aryloxide
complex to give a bis(imido) monomer23 and by a chromium(I)
complex with ꢀ-diketiminate ligands to give the bis(imido)-
bridged dimer.24 The reaction of ReOCl3(PPh3)2 with hydrazines
forms Re(NR)Cl3(PPh3)2.25 Scission of the N-N bond, however,
The goal of the present study is to discover N-N oxidative
addition processes that form oxidizing bis(amido) complexes,
which could be used in catalytic diamination reactions. We
report here the reactivity of di- and tetrasubstituted hydrazines
with three systems known to undergo facile oxidative addition
processes: IrCl(PEt3)3, Rh(NNN)Cl (NNN ) 2,6-(CHdNCy)2-
C5H3N), and (NN)PtMe2(NN ) phenanthroline or 2,2′-bipyri-
dine).
Results
I. Reaction of IrCl(PEt3)3 with PhNHNHPh. The 16-ele-
ctron Ir(I) trisphosphine complexes IrCl(PR3)3 (R ) Me, Et)
have been shown to oxidatively add B-B,26 O-H,27 and C-H28
bonds, among others.29 Of particular interest is the addition of
the aniline N-H bond to form the stable six-coordinate amido
hydride complex Ir(PEt3)3(NHPh)(H)Cl.29d The formation of the
stable Ir(III) anilide species suggests that an Ir(III) bis(anilide)
complex may also be accessible.
A solution of IrCl(PEt3)3, generated from [Ir(COE)2Cl]2 and
PEt3,30 reacts with excess 1,2-diphenylhydrazine (PhNHNHPh)
at 90 °C to give two major iridium-containing products. Both
products contain a cyclometalated azobenzene ligand: the 1:1
complex (Et3P)2Cl(H)Ir(C6H4NdNPh) (1) and the bimetallic 2:1
complex [(Et3P)2Cl(H)Ir]2(µ-C6H4NdNC6H4) (2) (eq 1). Com-
(14) (a) Du, H.; Zhao, B.; Shi, Y. J. Am. Chem. Soc. 2007, 129, 762–
763. (b) Du, H.; Yuan, W.; Zhao, B.; Shi, Y. J. Am. Chem. Soc. 2007, 129,
7496–7497. (c) Komatsu, M.; Tamabuchi, S.; Minakata, S.; Ohshiro, Y.
Heterocycles 1999, 50, 67–70.
(15) (a) Hitchcock, P. B.; Hughed, D. L.; Maguire, M. J.; Marjani, K.;
Richards, R. L. J. Chem. Soc., Dalton Trans. 1997, 4747–4752. (b) Kuwata,
S.; Mizobe, Y.; Hidai, M. Inorg. Chem. 1994, 33, 3619–3620. (c) Takei,
I.; Dohki, K.; Kobayashi, K.; Suzuki, T.; Hidai, M. Inorg. Chem. 2005, 44,
3768–3770. (d) Schollhammer, P.; Pétillon, F. Y.; Poder-Guillou, S.;
Saillard, J. Y.; Talrmin, J.; Muir, K. W. Chem. Commun. 1996, 2633–2634.
(e) Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 1999, 38,
1954–1955. (f) Malinak, S. M.; Demadis, K. D.; Coucouvanis, D. J. Am.
Chem. Soc. 1995, 117, 3126–3133. (g) Nakajima, Y.; Suzuki, H. Organo-
metallics 2003, 22, 959–969. (h) Nakajima, Y.; Inagaki, A.; Suzuki, H.
Organometallics 2004, 23, 4040–4046. (i) Nakajima, Y.; Suzuki, H.
Organometallics 2005, 24, 1860–1866.
(1)
(16) (a) Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Inorg. Chem. 1991,
30, 1143–1144. (b) Barry, J. T.; Chisholm, M. H.; Folting, K.; Huffman,
J. C.; Streib, W. E. Polyhedron 1997, 16, 2113–2133. (c) Gray, S. D.;
Thorman, J. L.; Adamian, V. A.; Kadish, K. M.; Woo, L. K. Inorg. Chem.
1998, 37, 1–4. (d) Aubart, M. A.; Bergman, R. G. Organometallics 1999,
18, 811–813. (e) Shaver, M. P.; Fryzuk, M. D. J. Am. Chem. Soc. 2005,
127, 500–501. (f) Watanabe, D.; Gondo, S.; Seino, H.; Mizobe, Y.
Organometallics 2007, 26, 4909–4920.
plexes 1 and 2 account for ∼60% of the starting Ir; their ratio
29b
and yields vary depending upon initial reaction conditions. Small
amounts (∼10–20%) of the dihydride complex Ir(PEt3)3Cl(H)2
are formed, in addition to several other hydride-containing
products (∼10–20%). In sealed NMR tube reactions, H2 is not
observed, but we estimate that even 100% yield of H2 would
(17) (a) Schrock, R. R.; Glassman, T. E.; Vale, M. G. J. Am. Chem.
Soc. 1991, 113, 725–726. (b) Schrock, R. R.; Glassman, T. E.; Vale, M. G.;
Kol, M. J. Am. Chem. Soc. 1993, 115, 1760–1772. (c) Vale, M. G.; Schrock,
R. R. Inorg. Chem. 1993, 33, 2767–2772. (d) Schrock, R. R. Acc. Chem.
Res. 2005, 38, 955–962.
1
have given a concentration barely at our detection limit by H
(26) Clegg, W.; Lawlor, F. J.; Marder, T. B.; Nguyen, R.; Norman, N. C.;
Orpen, A. G.; Quayle, M. J.; Rice, C. R.; Robins, E. G.; Scott, A. J.; Souza,
F. E. S.; Stringer, G.; Whittel, G. R. J. Chem. Soc., Dalton Trans. 1998,
301–309.
(18) Ohki, Y.; Takikawa, Y.; Hatanaka, T.; Tatsumi, K. Organometallics
2006, 25, 3111–3113.
(19) Verma, A. K.; Lee, S. C. J. Am. Chem. Soc. 1999, 121, 10838–
10839.
(27) Blum, O.; Milstein, D. J. Am. Chem. Soc. 2002, 124, 11456–11467.
(28) (a) Bleeke, J. R.; Haile, T.; Chiang, M. Y. Organometallics 1991,
10, 19–21. (b) Bleeke, J. R.; Ortwerth, M. F.; Chiang, M. Y. Organome-
tallics 1992, 11, 2740–2743. (c) Bleeke, J. R.; Haile, T.; New, P. R.; Chiang,
M. Y. Organometallics 1993, 12, 517–528. (d) Bleeke, J. R.; Hinkle, P. V.
Organometallics 2001, 20, 1939–1951.
(20) Hoover, J. M.; DiPasquale, A.; Mayer, J. M.; Michael, F. E.
Organometallics 2007, 26, 3297–3305.
(21) Yu, T.; Brennessel, W. W.; Holland, P. L. Organometallics 2007,
26, 3217–3226.
(22) Cohen, R.; Rybtchinski, B.; Gandelman, M.; Shimon, L. J. W.;
Martin, J. M. L.; Milstein, D. Angew. Chem., Int. Ed. 2003, 42, 1949–
1952.
(29) (a) Si-H: Simons, R. S.; Gallucci, J. C.; Tessier, C. A.; Youngs,
W. J. J. Organomet. Chem. 2002, 654, 224–228. (b) H2: Brunet, J.-J.; Diallo,
O.; Neibecker, D.; Roque, K. C. R. Acad. Sci., Ser. IIc: Chim. 1999, 2,
215–219. (c) B-X: Souza, F. E. S.; Nguen, P.; Marder, T. B.; Scott, A. J.;
Clegg, W. Inorg. Chim. Acta 2005, 358, 1501–1509. (d) N-H: Casalnuovo,
A. L.; Calabrese, J. C.; Milstein, D. J. Am. Chem. Soc. 1988, 110, 6738–
6744.
(23) Lentz, M. R.; Vilardo, J. S.; Lockwood, M. A.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 2004, 23, 329–343.
(24) (a) Tsai, Y.-C.; Wang, P.-Y.; Chen, S.-A.; Chen, J.-M. J. Am. Chem.
Soc. 2007, 129, 8066–8067. (b) Monillas, W. H.; Yap, G. P. A.; MacAdams,
L. A.; Theopold, K. H. J. Am. Chem. Soc. 2007, 129, 8090–8091.
(25) Arterburn, J. B.; Venkateswara, K.; Perry, M. C. Angew. Chem.,
Int. Ed. 2000, 39, 771–772.
(30) Bleeke, J. R.; Haile, T.; Chaing, M. Y. Organometallics 1991, 10,
19–21.

2240 Organometallics, Vol. 27, No. 10, 2008
HooVer et al.
NMR spectroscopy.31 PhNH2 is not observed over the course
of the reaction, but azobenzene (PhNdNPh) is observed in small
amounts and may be an intermediate in the formation of 1 and
2. Monitoring the reaction by 1H NMR spectroscopy indicates
that the ratio of Ir(PEt3)3Cl(H)2 to all the azobenzene-containing
species combined (1, 2, and PhNdNPh) remains constant over
the course of the reaction (Supporting Information S1). Treat-
ment of IrCl(PEt3)3 with PhNdNPh also forms complexes 1
and 2, without the formation of any other hydride-containing
species. Isolated 1 converts to bimetallic 2 upon heating with 2
equiv of IrCl(PEt3)3 in THF-d8. Related additions of azobenzene
to IrCl(PR3)332 or IrCl(PR3)2CO33 to give species analogous to
1 have been previously reported.
Characterization of (Et3P)2Cl(H)Ir(C6H4NdNPh) (1). Com-
plex 1 is pentane-soluble and was isolated by column chroma-
1
tography in low yield (∼20%). The H NMR spectrum of 1
has seven aryl resonances, indicating a cyclometalated ring. A
hydride resonance is observed at -17.8 ppm (t, 2JHP ) 17 Hz),
which is consistent with a hydride cis to two equivalent
phosphines. The methylene protons of the phosphine ethyl
groups appear as diastereotopic resonances, but 31P{1H} NMR
spectroscopy shows only a single resonance (δ ) -6.40). These
data are consistent with the isomer of 1 as shown in eq 1. These
spectroscopic data are in agreement with those of the known
triphenylphosphine analogue of 1.34 In addition, the isotope
pattern observed by ESI/MS confirms 1 as a monomer with m/z
) 647 [M + H]+.
Figure 1. ORTEP of [(Et3P)2Cl(H)Ir]2(µ-C6H4NdNC6H4) (2) with
50% probability ellipsoids. The hydride ligands were not located
in the structure (but presumably occupy the sites trans to N1 and
N1_3).
Table 1. Selected Bond Distances (Å) and Angles (deg) in 2
Ir1-C1
1.970(5)
2.219(4)
2.3193(13)
2.3224(13)
2.5074(11)
1.316(8)
C1-Ir1-N1_3
C1-Ir1-Cl1
P1-Ir1-P2
75.83(16)
176.87(12)
163.92(5)
Ir1-N1_3
Ir1-P1
Ir1-P2
Ir1-Cl1
N1-N1_3
N1-C2
Characterization of [(PEt3)2Cl(H)Ir]2(µ-C6H4NdNC6H4)
(2). The 2:1 complex 2 was isolated from the reaction mixture
by precipitation with pentane in 20% yield. Precipitation is more
selective when 2 is prepared from IrCl(PEt3)3 with PhNdNPh,
1.391(6)
1
Table 2. Crystallographic Data for 2 and 4
2
since this avoids the coprecipitation of PhNHNHPh. The H
4
NMR spectrum of 2 in THF-d8 shows only four aryl resonances
2
and a hydride resonance at δ ) -17.6 ppm (t, JHP ) 18 Hz)
formula
fw
C36H68Cl2Ir2N2P4
1108.14
P21/c
monoclinic
red
0.50 × 0.40 × 0.2
12.1403(4)
12.0878(3)
16.6594(4)
90
127.4751(12)
90
2100.64(10)
2
1.755
6.635
C17H18N4O2Pt
505.44
P21/c
monoclinic
yellow
0.10 × 0.10 × 0.05
14.4886(2)
22.0667(4)
10.2932(3)
90
105.1153(7)
90
3177.04(12)
8
indicating coupling to two equivalent cis phosphines. This is
confirmed by a single resonance in the 31P{1H} NMR spectrum
(δ ) -4.37 ppm). Complex 2 is bimetallic in solution, as
indicated by the isotope pattern observed by ESI/MS (m/z )
955, [M - (Cl + PEt3)]+). Although cyclometalated azobenzene
complexes typically contain a single metal center like 1, a
bimetallic structure has been reported for the Cp*Ir system
[Cp*IrBr]2(µ-C6H4N-NC6H4).35
space group
cryst syst
cryst color
cryst dimen (mm)
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Crystals of 2 suitable for X-ray diffraction analysis were
grown from vapor diffusion of pentane into a solution of 2 in
THF and confirm a bimetallic species in which the two halves
are related by an inversion center, as shown in Figure 1 (the
Z
dcalc (mg m-3
)
2.113
8.851
1936
µcalc (mm-1
F(000)
)
1092
no. of unique rflns
no. of data used
no. of params refined
Ra
5154
5154
214
0.0383
0.0991
0.990
8181
8181
437
0.0497
0.1103
0.916
(31) An NMR tube containing 1 atm of H2 (1.9 mL, 0.08 mmol) and
500 µL of THF-d8 was found to have a concentration of 19 mM of H2 in
solution (by NMR integration vs a standard). Our conditions typically
include 10–15 µmol of IrCl(PEt3)3; forming 10–15 µmol of H2 would mean
a soluble concentration of 2.5–3.7 mM. Thus the concentrations of H2
formed in these reactions would be difficult to observe by 1H NMR
spectroscopy.
b
Rw
GOFc
/
1 2
2
2
a R ) ∑|Fo| - |Fc|/∑Fo. b Rw ) [ w(|Fo| - |Fc|) ⁄ w|Fo| ]
; w )
∑
∑
.
(32) Huang, L.-Y.; Aulwurm, U. R.; Heinemann, F. W.; Knoch, F.;
Kisch, H. Chem.-Eur. J. 1998, 4, 1641–1646.
/
1/σ2(Fo). GOF ) [ w(|Fo| - |Fc|) ⁄(Nobs - Nparms)]1
2
2
c
∑
(33) Bruce, M. I.; Goodall, B. L.; Stone, G. A.; Thomson, B. J. Aust.
J. Chem. 1974, 27, 2135–2138.
metric and crystallographic data are shown in Tables 1 and 2).
Each iridium center is octahedral with a hydride ligand, not
seen in the structure, occupying the sixth site. The azobenzene
has a chelate angle of 75.83(16)° and an N-N distance (1.316(8)
Å) longer than that in both the monomeric PPh3 analogue
(PPh3)2Cl(H)Ir(C6H4N)NPh (1.269(20) Å)34 and a related Cp*Ir
dimer [Cp*IrBr]2(µC6H4NdNC6H4) (1.247(21) Å), but is much
shorter than that of the hydrazido-bridged dimer [Cp*Ir]2(µ-
C6H4N-NC6H4) (1.419 Å).35 The small torsion angle (N1_3-
(34) Van Baar, J. F.; Vrieze, K.; Stufkens, D. J. J. Organomet. Chem.
1975, 85, 249–263.
(35) Zambrano, C. H.; Sharp, P. R.; Barnes, C. L. Organometallics 1995,
14, 3607–3610.
(36) Van Baar, J. F.; Meii, R.; Olie, K. Cryst. Struct. Commun. 1974,
3, 587–593.
(37) (a) Koga, G.; Warner, J. C.; Anselme, J.-P. Radical reactions of
azo, hydrazo and azoxy compounds. In The Chemistry of Hydrazo, Azo
and Azoxy Groups; Patai, S., Ed.; John Wiley & Sons, Ltd.: London, 1975;
Vol. 2, pp 627-634. (b) Musso, H. Chem. Ber. 1959, 92, 2881–2886. (c)
Welzel, P. Chem. Ber. 1970, 103, 1318–1333.

ReactiVity of Low-Valent Ir, Rh, and Pt Complexes
Organometallics, Vol. 27, No. 10, 2008 2241
Chart 1. Tetrasubstituted Hydrazines Examined
II. Reactivity of Rh(NNN)Cl with Hydrazines. Square-
planar Rh(I) complexes containing tridentate “pincer” nitrogen
donor ligands, Rh(2,6-(RNdCR′)2C5H3N)Cl, have been de-
scribed by Vrieze and co-workers.38 These compounds undergo
rapid oxidative addition of a variety of C-Cl bonds under mild
conditions. The hemilabile nature of the imine arm provides
accessible coordination sites on the Rh(I) complex, while the
tridentate binding mode is available to stabilize the Rh(III)
product. One derivative (R ) i-Pr, R′ ) Me) has been shown
to react with O2 to form a stable side-on bound peroxo species.39
Heating a mixture of 8 equiv of PhNHNHPh and Rh(NNN)Cl
(NNN ) 2,6-(CyNdCH)2C5H3N) in toluene-d8 at temperatures
up to 125 °C results in no change in the rhodium complex, and
only the conversion of PhNHNHPh (2.4 equiv after 8 h) to
1
PhNdNPh and PhNH2 is observed by H NMR spectroscopy
(eq 2). Under the same conditions in the absence of the rhodium
complex, <5% yields of PhNdNPh and PhNH2 are observed.
This is consistent with literature reports that the disproportion-
ation of PhNHNHPh occurs thermally only at higher temper-
atures.40 In sum, Rh(NNN)Cl shows limited reactivity with
PhNHNHPh and oxidative addition of the N-N bond has not
been observed.
N1-C2-C1 ) -2.6(6)°) indicates a planar azobenzene ligand.
The Ir-N bond length (2.219(4) Å) is longer than that of both
the monomeric PPh3 complex (2.148(14) Å)34 and the Cp*-
ligated dimer (2.041(10) Å).35 The Ir-C bond length (1.970(5)
Å) is similar to that in both the monomeric PPh3 analogue
(2.002(19) Å)36 and the [Cp*IrBr]2(µ-C6H4NdNC6H4) system
(2.023(14) Å).35
Competition between PhNHNHPh and PhNdNPh in
the Formation of 1. The formation of the cyclometalated
iridium complexes 1 and 2 as the major products in the reaction
of IrCl(PEt3)3 with PhNHNHPh was unexpected. To determine
if PhNHNHPh is oxidized to PhNdNPh before cyclometalation,
IrCl(PEt3)3 was combined with an equimolar mixture of either
PhNHNHPh and TolNdNTol, or TolNHNHTol and PhNdNPh,
and heated in THF-d8 at 90 °C. The reactions formed 1 and its
tolyl analogue (Et3P)2Cl(H)Ir((CH3)C6H3NdNTol) (1-Tol) in
ratios of 1:3.6 and 4.5:1, respectively (Figures S2 and S3). Both
reactions formed only minor amounts of the bimetallic products
2 and 2-Tol[(Et3P)2Cl(H)Ir]2(µ-CH3C6H3NdNC6H3CH3). In
both experiments, the cyclometalated complex derived from the
azo compound is formed more rapidly than that from the
hydrazine. Thus the azo compound is likely to be an intermediate
in the reaction of IrCl(PEt3)3 with PhNHNHPh.
(2)
Rh(NNN)Cl is unreactive with both Ph2NNPh2 (∼3 equiv)
and Ph(Me)NN(Me)Ph (∼6 equiv) at 50 °C, 80 °C, and even
heating at 125 °C in toluene-d8 for 14 h. Under these conditions,
the hydrazine decomposes to the amine (diphenylamine and
N-methylaniline respectively) and the rhodium starting material
remains unchanged. Similarly, after heating an excess of each
of the more electron-deficient hydrazines (bisuccinimide, biph-
thalimide, and N-dimethylaminophthalimide) with Rh(NNN)Cl
in toluene-d8 no reaction was observed by 1H NMR spectroscopy
at temperatures up to 125 °C.
III. Reactivity of (NN)PtMe2 with Hydrazines. Dimeth-
ylplatinum complexes with nitrogen donor ligands have been
shown to oxidatively add a number of species including those
with C-X, C-H, C-O, O-H, and Sn-H bonds.41 Of particular
interest are the oxidative additions of O-O, S-S, and Se-Se
bonds by the phenanthroline complex (phen)PtMe2 to give the
six-coordinate (phen)Pt(Me)2(RE)2 species.42 The preference of
Reactivity of IrCl(PEt3)3 with Tetrasubstituted Hydra-
zines. To avoid the formation of the azo complexes, IrCl(PEt3)3
was treated with the tetrasubstituted hydrazines shown in Chart
1. Heating a mixture of IrCl(PEt3)3 with tetraphenylhydrazine
(Ph2NNPh2) at 70 °C in THF-d8 leads only to formation of
diphenylamine, with no change in the Ir starting material as
indicated by 31P NMR spectroscopy. Control experiments show
that Ph2NNPh2 converts to Ph2NH under these conditions in
the absence of the iridium complex. The source of H is not
clear; a Ph2NH resonance is observed at δ ) 7.3 ppm in the 1H
NMR spectra in THF-d8, and this has likely undergone exchange
with the residual H2O in the solvent. The thermal decomposition
of Ph2NNPh2 to form Ph2NH and other rearrangement products
is known.37 Similarly, 1,2-diphenyl-1,2-dimethylhydrazine
(PhMeN-NMePh) does not react with IrCl(PEt3)3 up to 90 °C.
The more electron-deficient tetrasubstituted hydrazines bisuc-
cinimide, biphthalimide, and N-dimethylaminophthalimide (Chart
1b) also did not react with IrCl(PEt3)3. In all cases, the
IrCl(PEt3)3 starting material is observed by 31P{1H} NMR
spectroscopy after ∼30 h at 90 °C. Under these conditions
(IrCl(PEt3)3, 70 °C, THF-d8) oxidative addition of the N-H
bond of Ph2NH is not observed over the course of 2 days or at
higher temperatures (90 °C) for a day, although addition of the
N-H bond of PhNH2 occurs readily under milder conditions
(50 °C).29d
these systems to react with the O-O bond in H
2O2 rather than
the O-H bond indicates that protonolysis can be slower than
(38) (a) Hendrikus, F. H.; Ernsting, J. M.; Kraneburg, M.; Kooijman,
H.; Veldman, N.; Spek, A. L.; van Leeuwen, P. W. N. M.; Vrieze, K.
Organometallics 1997, 16, 887–900. (b) Haarman, H. F.; Kaagman, J.-
W. F.; Smeets, W. J. J.; Spek, A. L.; Vrieze, K. Inorg. Chim. Acta 1998,
270, 34–45.
(39) Haarman, H. F.; Bregman, F. R.; van Leeuwen, P. W. N. M.; Vrieze,
K. Organometallics 1997, 16, 979–985.
(40) (a) Koga, G.; Koga, N.; Anselme, J.-P. Radical reactions of azo,
hydrazo and azoxy compounds In The Chemistry of Hydrazo, Azo and Azoxy
Groups; Patai, S., Ed.; John Wiley & Sons, Ltd.: London, 1975; Vol. 1,
Part 1, pp 921–922. (b) Stieglitz, J.; Graham, H. T. J. Am. Chem. Soc.
1916, 38, 1736–60. (c) Holt, P. F.; Hughes, B. P. J. Chem. Soc. 1953, 1666–
1669.
(41) Rendina, L. M.; Puddephatt, R. J. Chem. ReV. 1997, 97, 1735–
1754.
(42) Aye, K.-T.; Vittal, J. J.; Puddephatt, R. J. J. Chem. Soc., Dalton
Trans. 1993, 1835–1839.

2242 Organometallics, Vol. 27, No. 10, 2008
HooVer et al.
oxidative addition in this system. By analogy, N-N bond
activation could be observed instead of N-H cleavage in the
analogous reactions with hydrazines.
Addition of PhNHNHPh (∼10 equiv) to a solution of
(phen)PtMe2 in acetone-d6 shows no change in the (phen)PtMe2
1
resonances of the H NMR spectrum after heating at 125 °C
for 20 h. In contrast, the 2,2′-bipyridine analogue (bpy)PtMe2
reacts with PhNHNHPh (4 equiv) at 80 °C to form CH4 and an
intractable yellow precipitate.
To eliminate the possible N-H addition pathway, the
tetrasubstituted hydrazines were studied. Heating a mixture of
(bpy)PtMe2 with Ph2NNPh2 in acetone-d6 to 80 °C leads to the
formation of Ph2NH before any change is observed in the Pt
species. (bpy)PtMe2 is unreactive toward each of the electron-
deficient hydrazines shown in Chart 1b (4 equiv) in DMF-d7
up to 125 °C; only decomposition of the platinum starting
material is observed.
Figure 2. ORTEP of (bpy)Pt(η2–AcN–NC(O)(CH2)2CHCH2 (4)
Characterization of (bpy)Pt(η2-AcN-NC(O)(CH2)2CHC-
showing one of the two independent molecules in the unit cell with
50% probability ellipsoids.
H2 (4). In our efforts to use N-N oxidative addition to develop
a catalytic amination reaction, we investigated the reactivity of
(bpy)PtMe2 with a hydrazine containing a pendant alkene:
AcNHNHC(O)CH2CH2CHdCH2 (3). Heating a solution of
(bpy)PtMe2 with 3 in acetone at 80 °C for 1 day leads to
formation of CH4 and precipitation of a new platinum product
(4) (eq 3). ESI/MS analysis (m/z ) 506) is consistent with the
incorporation of a single hydrazine and loss of 2 equiv of CH4.
The 1H NMR spectrum of 4 contains eight aryl resonances and
seven distinct resonances in the alkyl region due to the
hydrazine. This indicates that both the C2V symmetry of the bpy
ligand and the Cs symmetry of the hydrazine have been broken.
The coupling of the alkyl protons is consistent with cyclization
to the metallacycle 4 (eq 3).
Table 3. Selected Bond Distances (Å) and Angles (deg) in 4
Pt1-N1
Pt1-N4
Pt1-N3
Pt1-C1
N1-N2
2.007(6)
2.085(6)
2.022(6)
2.045(7)
1.394(8)
N1-Pt1-C1
N1-Pt1-N4
81.8(3)
100.7(2)
(2.067(12) Å).45 The Pt1-N4 distance is 0.063 Å longer than
the Pt1-N3 distance, consistent with the stronger trans influence
of the alkyl ligand compared to the amide ligand.
The metallacycle 4 results from cleavage of both N-H bonds
of the hydrazine and addition of one hydrazine nitrogen atom
to the alkene to form a new C-N bond. The N-N bond remains
intact. A related metallacycle (a fused four- and five-membered
ring system) has been isolated by Widenhoefer et al.46 This
complex is derived from an alkene-tethered amine and
PtCl2(PPh3)2 and forms through attack of the amine on the
coordinated alkene. Widenhoefer’s metallacycle involves co-
ordination of a neutral amine, while in 4 the platinum
coordinates an anionic amide nitrogen. Further studies of
hydrohydrazination by (bpy)PtMe2 are in progress.
(3)
The structure of 4 was confirmed by crystallographic struc-
tural analysis. X-ray quality crystals of 4 formed in the above
reaction mixture after heating at 80 °C. The product crystallized
with two independent molecules in the unit cell; one of these is
shown in Figure 2 (crystallographic and metric data are shown
in Tables 2 and 3). The binding mode of the substrate indicates
net cleavage of both N-H bonds and cyclization of the alkene
tether to form the five-membered pyrrolidinone, which after
protonation would yield the hydrohydrazination product. The
platinum center is nearly square planar with chelating bite angles
of 81.8° (C1-Pt1-N1) and 79.3° (N3-Pt1-N4). The N-N
distance of 1.394(8) Å is consistent with a single bond and is
only slightly shorter than those seen in chelating benzoyl- and
acyl-hydrazido platinum complexes (1.401(9)43 and 1.421(6)44
Å, respectively). The Pt-N(amide) bond distance (Pt1-N1,
2.007(6) Å) is slightly shorter than those reported for the acyl-
(2.050(5) Å) and benzoylhydrazido species (2.047(6) Å). The
Pt-C bond length (2.045(7) Å) is similar to that in a complex
containing a related ring system with a neutral chelating N
Discussion
Overall, the formation of 1 and 2 from IrCl(PEt3)3 and
PhNHNHPh (eq 1) involves oxidation of both the hydrazine
and the iridium center. This could occur through either initial
N-H addition or initial C-H addition. While the data available
do not rule out initial C-H activation, the likely intermediacy
of PhNdNPh suggests initial N-H addition to give an η1-
hydrazido(1-) complex. This is analogous to the reported
oxidative addition of the N-H bond of PhNH2 to IrCl(PEt3)3
which occurs readily under milder conditions (50 °C).29d The
hydrazido complex can then lose H2 to form azobenzene,
followed by cyclometalation to give 1, which then reacts further
with IrCl(PEt3)3 to form 2 (Scheme 1). An alternative pathway
involving ꢀ-hydride elimination to give IrCl(PEt3)3H2 seems
inconsistent with the low yield of this product unless PhNdNPh
promotes its reductive elimination of H2.
(45) Briggs, J. R.; Crocker, C.; McDonald, W. S.; Shaw, B. L. J. Chem.
Soc., Dalton Trans. 1982, 457–463.
(43) Ittel, S. D.; Ibers, J. A. Inorg. Chem. 1973, 12, 2290–2295.
(44) Chan, D.; Cronin, L.; Duckett, S. B.; Hupfield, P.; Perutz, R. N.
New J. Chem. 1998, 511–516.
(46) Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2005, 127,
1070–1071.

ReactiVity of Low-Valent Ir, Rh, and Pt Complexes
Organometallics, Vol. 27, No. 10, 2008 2243
Scheme 1. Plausible Pathway for the Formation of 1 and 2
upon heating a solution of Cp*(PMe3)2RuNPh2, which likely
occurs by homolysis.51 Polyamido complexes are well known
for the early transition metals,52 but are rare with later metals.
Of the late metal amido complexes that are known,53 a few
bis(amido) species have been isolated54 including an anionic
Pd(II) bis(anilide) salt,55 Ru(IV) and Ru(III) complexes with
chelating nitrogen ligands,56,57 and Cp*IrIII(NHTs)2(PPh3).58
If the rarity of higher oxidation state bis(amido) complexes
of the late metals is due to their instability, then the difficulty
in observing N-N oxidative addition could have a thermo-
chemical origin. An estimate of the thermochemistry of hydra-
zine oxidative addition can be derived for the IrCl(PEt3)3 system.
We use the relative M-H and M-N bond strengths estimated
by Bryndza et al., that MsH are ca. 21 kcal mol-1 stronger
than M-N, derived from equilibrium exchange experiments of
Cp*(PMe3)2RuX (X ) OH, H, NHPh).51 IrCl(PEt3)3 rapidly
oxidatively adds aniline to give the amido hydride complex,
The Rh(NNN)Cl system reacts with PhNHNHPh to yield only
the disproportionation products PhNdNPh and PhNH2. Dis-
proportionation of PhNHNHPh is known to occur at high
temperatures (∼240–280 °C),40 and we and others have found
this process to be catalyzed by metal complexes under milder
conditions,15a–c although this catalysis is not well understood.
The reaction of (bpy)PtMe2 with hydrazine 3 led to the
isolation of the new Pt metallacycle 4 (eq 3). A plausible
mechanism for metallacycle formation begins with the cleav-
age of the N-H bond and loss of CH4. PtCl2(PR3)2 complexes
are known to eliminate both N-H hydrogens of electron-
deficient hydrazides (acetyl and benzoyl protected) to give stable
square-planar Pt(II) hydrazido(2-) species with loss of HCl.47
These compounds can also be obtained from reaction of
Pt(PR3)2(H2CdCH2) with the corresponding azo compounds.48
[Pt(dppp)(OH)]2 (dppp ) diphenylphosphinopropane) reacts
with 1,1- and 1,2-disubstituted hydrazines to give the corre-
sponding hydrazido complexes, where deprotonation of the
hydrazine is likely assisted by the basic hydroxo ligands.49
Cleavage of N-N vs N-H Bonds. N-N bond cleavage has
been observed, as noted above, in strongly reducing early
transition metal systems and in a number of multimetallic and
metal hydride complexes. In the systems described here,
however, which undergo a variety of facile X-Y oxidative
addition reactions, N-N bond scission is not favored. Instead,
the hydrazines react through different pathways. IrCl(PEt3)3
reacts with PhNHNHPh to form the cyclometalated azobenzene
complexes 1 and 2. Reaction of (bpy)PtMe2 with the alkene-
tethered hydrazide 3 gives an alkyl-amido metallacycle 4,
formed through cleavage of both N-H bonds. The Rh(NNN)Cl
complex is unchanged by reaction with PhNHNHPh, although
this system promotes the disproportionation of PhNHNHPh into
PhNH2 and PhNdNPh via an unknown mechanism. None of
these systems react with the tetrasubstituted hydrazines.
Thermodynamic Factors. The inertness of the N-N bond
is surprising given the homolytic bond energies. The N-N bond
of a hydrazine is by far the weakest bond in the molecule: for
H2NNH2, D(N-N) ) 65 kcal mol-1 and D(N-H) ) ∼80
kcal.50 Although the N-N bond is weak, it is possible that
formation of a bis(amido) species is thermodynamically unfa-
vorable. The only example of formation of a hydrazine from a
metal amide that we are aware of is the production of Ph2NNPh2
cleaving the N-H bond (92 kcal mol-1 50
and forming
)
Ir-NHPh and Ir-H bonds. The oxidative addition of PhHN-
NHPh would cleave the much weaker N-N bond, only 34 kcal
mol-1 59
and form two Ir-NHPh bonds. Based on Bryndza’s
,
estimate, the Ir-NHPh bond should be about 21 kcal mol-1
weaker than the Ir-H. Overall, however, cleavage of the 58
kcal mol-1 weaker bond (N-N vs N-H) is a much larger effect
than the formation of the 21 kcal mol-1 weaker bond (Ir-NHPh
vs Ir-H). Thus PhHN-NHPh oxidative addition is indicated
to be more favorable than the facile H-NHPh addition by a
remarkable 37 kcal mol-1 60
. Clearly, the inertness of the N-N
bond of hydrazines to IrCl(PEt3)3 does not have a thermody-
namic origin.
With (phen)PtMe2, however, the facile addition of HOOH
mentioned above does not imply that PhHN-NHPh addition
is necessarily favorable. The two Pt-OH bonds should each
be ca. 27 kcal mol-1 stronger than the Pt-NHR bonds,61 more
than enough to compensate for the stronger O-O bond of
HOOH (51 kcal mol-1) relative to PhHN-NHPh (34 kcal
mol-1), making N-N addition 37 kcal mol-1 less favorable
than O-O addition. Still, given the weakness of the hydrazine
(51) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Bercaw, J. E. J. Am.
Chem. Soc. 1987, 109, 1444–1456.
(52) (a) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C.
Amides of Transition Metals and f-Block Metals: Synthesis, Physical
Properties, and Structures. In Metal and Metalloid Amides; John Wiley &
Sons, Ltd.: New York, 1980; pp 465–544. (b) Bradley, D. C.; Chisholm,
M. H. Acc. Chem. Res. 1976, 9, 273–280.
(53) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163–1188.
(54) Martin, G. C.; Palenik, G. J.; Boncella, J. M. Inorg. Chem. 1990,
29, 2027–2030.
(55) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1997, 119, 8232–
8245.
(56) (a) Huang, J.-S.; Sun, X.-R.; Leung, S. K.-Y.; Cheung, K.-K.; Che,
C.-M. Chem.-Eur. J. 2000, 6, 334–344. (b) Leung, S. K.-Y.; Huang, J.-S.;
Zhu, N.; Che, C.-M. Inorg. Chem. 2003, 42, 7266–7272. (c) Chiu, W.-H.;
Peng, S.-M.; Che, C.-M. Inorg. Chem. 1996, 35, 3369–3374.
(57) Au, S.-M.; Zhang, S.-B.; Fung, W.-H.; Yu, W.-Y.; Che, C.-M.;
Cheung, K.-K. Chem. Commun. 1998, 2677–2678.
(58) Ishiwata, K.; Kuwata, S.; Ikariya, T. Dalton Trans. 2007, 3606–
3608.
(59) Shaw, R. Thermochemistry of hydrazo, azo and azoxy groups In
The Chemistry of Hydrazo, Azo and Azoxy Groups; John Wiley & Sons,
Ltd.: London, 1975; Part 1, pp 53–68.
(47) Dilworth, J. R.; Kasenally, A. S.; Hussein, F. M. J. Organomet.
Chem. 1973, 60, 203–207.
(48) Chan, D.; Cronin, L.; Duckett, S. B.; Hupfield, P.; Perutz, R. N.
New J. Chem. 1998, 511–516.
(60) If the linear correlation of M-X and H-X bond strengths is used,
the M-H bond is estimated to be 12 kcal mol-1 weaker than the M-
NHPh. This analysis shows PhHN-NHPh oxidative addition to be 46 kcal
mol-1 more favorable than H-NHPh addition.
(49) Xia, A.; Sharp, P. R. Inorg. Chem. 2001, 40, 4016–4021.
(50) (a) Zhao, Y.; Bordwell, F. G.; Cheng, J.-P.; Wang, D. J. Am. Chem.
Soc. 1997, 119, 9125–9129. (b) N-N BDE of hydrazine: Gibson, S. T.;
Greene, J. P.; Berkowitz, J. J. Chem. Phys. 1995, 83, 4319–4328.
(61) D(H-OH) ) 119.2 kcal mol-1: Reints, W.; Pratt, D. A.; Korth,
H.-G.; Mulder, P. J. Phys. Chem. A 2000, 104, 10713–10720.

2244 Organometallics, Vol. 27, No. 10, 2008
HooVer et al.
N-N bond, with minimal 25 kcal mol-1 Pt-NHPh bonds62
Rh(NNN)Cl, and (NN)PtMe2. These systems were chosen
because among them they undergo a very wide range of
oxidative addition reactions. However, no N-N oxidative
addition process has been observed. Disubstituted hydrazines
were observed in the Ir and Pt systems to preferentially react
via the N-H bond: the iridium complex forming a cyclometa-
lated azo complex and the platinum one a metallahydrazination
product. Rh(NNN)Cl promotes the disproportionation of Ph-
NHNHPh into PhNdNPh and PhNH2. No reactivity was
observed with any of the systems with tetrasubstituted hydra-
zines, either electron-rich or electron-deficient derivatives. We
conclude that the oxidative addition of the N-N bond of
hydrazines is not a facile process and suggest that this has a
kinetic and not thermodynamic origin. Oxidative addition of
N-N bonds appears to be analogous to C-C oxidative addition,
which is also typically kinetically inhibited.
oxidative addition would be exothermic by ca. 16 kcal mol-1
.
This analysis suggests that the inability to observe hydrazine
oxidative addition does not have a thermochemical origin. While
such thermochemical estimates based on bond strength argu-
ments are necessarily approximate, the magnitude of the
predicted differences (16 to 37 kcal mol-1) provides a measure
of confidence in the conclusions.
Kinetic Factors. The metal complexes examined here are
unreactive with the hydrazine N-N bond most likely due to
high kinetic barriers. In this respect, N-N oxidative addition
may be similar to C-C bond oxidative addition.63 C-C
oxidative addition is typically observed only for highly strained
substrates,8 and one of the few possible examples of hydrazine
addition is with a highly strained diaziridinone.14 In cases where
oxidative addition of a nonstrained C-C bond would be
thermodynamically favorable, C-H activation usually occurs
instead.64 This is thought to be due at least in part to steric
difficulties in bringing the C-C bond to a metal center. Forming
monomeric η2-bound hydrazine complexes appears to be
difficult as well, as only a limited number have been isolated
These are most commonly obtained by protonation of the
corresponding hydrazido species,65 although a few exceptions
include cyclopentadiene-ligated Mo and W complexes,66 a Co
species with a tripod phsophine ligand [(CH3C(CH2PPh2)3)Co(η2-
N2H4)][BPh4],67 andtherecentlyreportedcis-[Fe(DMeOPrPE)2(η2-
N2H4)][BPh4]2 (DMeOPrPE ) 1,2-bis[bis(methoxypropyl)phos-
phino]ethane).68 Hydrazine η1- or bridging η2-binding modes
are more common.65
For tetrasubstituted hydrazines, steric factors likely contribute
to the difficulty in oxidative addition. While N-H oxidative
addition to IrCl(PEt3)3 is facile for aniline, we observe no
reaction with Ph2NH even at high temperatures. Similarly, the
reaction of (Cp*RuCl)4 with Ph2NNPh2 is much slower than
the analogous reaction with PhNHNHPh that occurs at room
temperature.69 Thus both the intrinsic inaccessibility of the
hydrazine N-N bond and other steric factors seem to contribute
to the high-energy transition state for oxidative addition of the
nonpolar N-N bond.
Experimental Section
General Considerations. All manipulations were performed
under N2 using standard high-vacuum-line or inert atmosphere
glovebox techniques unless otherwise noted. Solvents were pur-
chased from Fisher and Cambridge (deuterated) and were dried
before use (pentane, THF, THF-d8, toluene-d8 using sodium/
benzophenone; CHCl3 using CaH2; and acetone, acetone-d6 using
4 Å mol sieves). DMF-d7 was used as received. IrCl3 · 3H2O,
RhCl3 · 3H2O, and K2PtCl4 (Pressure Chemical) were used as
received. IrCl(PEt3)3,30 RhCl(2,6-(CyNdCH)2C5H3N),38a Pt(bpy)-
(CH3)2,70 Pt(phen)(CH3)2,71 TolNHNHTol,72 Ph2NNPh2,73 Ph(Me)-
NN(Me)Ph,74 bisuccinimide,75 biphthalimide,76 and N-dimethyl-
aminophthalimide77 were made according to literature procedures.
1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on Bruker
Avance 300 (1H and 31P{1H}) or 500 (1H, 13C{1H}) MHz
spectrometers and referenced to the residual solvent signal (1H and
13C) or an external H3PO4 standard (31P); all coupling constants
are reported in Hz. Assignments of the 13C{1H} NMR resonances
were made using a 2D HMQC experiment. Mass spectra were
recorded from CH3CN solutions on a Bruker Esquire ion trap
electrospray ionization mass spectrometer (ESI/MS). Elemental
analyses were performed by Atlantic Microlab, Inc. (Norcross, GA).
Typical Procedure for NMR Reactions with Hydrazines. All
reactions of metal complexes with hydrazines were performed in
J. Young NMR tubes under N2 with dry solvent, heated in an oil
bath, and removed to record 1H and 31P{1H} NMR spectra at room
temperature.
Conclusion
The reactivity of hydrazines has been examined with three
different low-valent, electron-rich complexes: IrCl(PEt3)3,
(PEt3)2Cl(H)Ir(C6H4NdNPh) (1). Compounds 1 and 2 can be
prepared from Ir and PhNHNHPh, but purification is easier when
azobenzene is used. PEt3 (175 µL, 1.18 mmol) was added to a
solution of [Ir(COE)2Cl]2 (72.5 mg, 0.081 mmol) in ether (30 mL)
at -78 °C. The reaction mixture was allowed to warm to RT and
stirred for 1 h. The solvent was removed and the resulting oil was
transferred to a sealed tube and combined with PhNdNPh (31.2
mg, 0.17 mmol) and THF (2 mL). After heating at 90 °C for 3
days, the solvent was removed and the crude solids were washed
with pentane (3 × 1 mL). The red pentane solution contains mostly
(62) If PtIV-NHPh bonds were weaker than 30 kcal mol-1, facile
homolysis would be observed, contrary to the behavior of such species (see
ref 11c). We estimate that the PtIII-NHPh bond is unlikely to be weaker
than 20 kcal mol-1, so the average Pt-N bond strength in the hypothetical
Pt(NHPh)2 species is most likely >25 kcal mol-1
.
(63) We thank Professor B. Scott Williams for his insightful suggestion.
(64) (a) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein, D. J. Am.
Chem. Soc. 1996, 118, 12406–12415. (b) Wick, D. D.; Northcutt, T. O.;
Lachicotte, R. J.; Jones, W. D. Organometallics 1998, 17, 4484–4492. (c)
Rybtchinski, B.; Oevers, S.; Montag, M.; Vigalok, A.; Rozenberg, H.;
Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2001, 123, 9064–9077.
(d) Ruhland, K.; Herdtweck, E. AdV. Synth. Catal. 2005, 347, 398–404.
(65) Heaton, B. T.; Jacob, C.; Page, P. Coord. Chem. ReV. 1996, 154,
193–229.
(70) Britovsek, G. J. P.; Taylor, R. A.; Sunley, G. J.; Law, D. J.; White,
A. J. P. Organometallics 2006, 25, 2074–2079.
(71) Monaghan, P. K.; Puddephatt, R. J. Organometallics 1984, 3, 444–
449.
(72) Khurana, J. M.; Singh, S. J. Chem. Soc., Perkin Trans. 1 1999,
1893–1895.
(73) Wieland, H.; Gambarjan, S. Ber. 1906, 39, 1499–1506.
(74) Katritzky, A. R.; Wu, J.; Verin, S. V. Synthesis 1995, 651–653.
(75) Hedaya, E.; Hinman, R. L.; Theodoropulos, S. J. Am. Chem. Soc.
1963, 85, 3052–3054.
(76) Drew, H. D.; Hatt, H. H. J. Chem. Soc. 1937, 16–26.
(77) Drew, H. D.; Hatt, H. H.; Hobart, F. A. J. Chem. Soc. 1937, 33–
37.
(66) (a) Bailey, N. A.; Frisch, P. D.; McCleverty, J. A.; Walker, N. W.;
Williams, J. J. Chem. Soc., Chem. Commun. 1975, 350–351. (b) Frisch,
P. D.; Hunt, M. M.; Kita, W. G.; McCleverty, J. A.; Rae, E.; Seddon, D.;
Swann, D.; Williams, J. J. Chem. Soc., Dalton Trans. 1979, 1819–1830.
(67) Vogel, S.; Barth, A.; Huttner, G.; Klein, T.; Zsolnai, L.; Kremer,
R. Angew. Chem., Int. Ed. Engl. 1991, 30, 303–304.
(68) Crossland, J. L.; Zakharov, L. N.; Tyler, D. R. Inorg. Chem. 2007,
46, 10476–10478.
(69) Reaction of (Cp*RuCl)4 with Ph2NNPh2 forms white solids
consistent with η6-arene coordination: Hoover, J. M. Unpublished results
(see Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J. Am. Chem. Soc. 1989,
111, 1698–1719. ).

ReactiVity of Low-Valent Ir, Rh, and Pt Complexes
Organometallics, Vol. 27, No. 10, 2008 2245
1, which can be purified by silica gel column chromatography
(hexanes/CH2Cl2, increasing CH2Cl2 as each product elutes) to give
20.6 mg of a red solid (0.032 mmol, 20%). 1H NMR (THF-d8, 500
MHz): δ 8.76 (d, J ) 7.5, 2H, o), 8.10 (d, J ) 7.5, 1H, H3), 7.80
(d, J ) 8, 1H, H6), 7.42 (t, J ) 8, 2H, m), 7.30 (t, J ) 7.5, 1H, p),
7.05 (t, J ) 6.5, 1H, H4), 6.88 (t, J ) 6.5, 1H, H5), 1.57 (m, 6H,
PCH2), 1.49 (m, 6H, PCH2), 0.71 (quint, 18H, CH3), –17.79 (t, J
) 17.5, 1H, IrH). 31P{1H} NMR (THF-d8, 121 MHz): δ -6.40
(s). 13C{1H} NMR (THF-d8, 125 MHz): δ 165.1 (C2), 156.8 (t, J
) 6.5, C1), 141.5 (C6), 132.7 (C5), 131.9 (C3), 130.6 (p), 128.9
(m), 126.7 (o), 122.0 (C4), 16.5 (t, J ) 17.5, PCH2), 7.6 (CH3).
ESI/MS (CH3CN): m/z 647 [M + H]+, 611 [M - Cl]+.
additional 45 min. The solids were filtered and washed with CHCl3.
The solvent was removed from the filtrate to give a white solid
(1.95 g, 12.5 mmol, 85% yield). The product was recrystallized
1
from ethyl acetate/hexanes. H NMR (CD2Cl2, 300 MHz): δ 8.63
(s, 1H, NH), 8.56 (s, 1H, NH), 5.82 (m, 1H CH), 5.07 (dd, J )
17,1.2, 1H dCH2), 5.00 (d, J ) 5, 1H, dCH2), 2.36 (m, 4H,
-CH2-), 2.01 (s, 3H, CH3). 13C{1H} NMR (CD2Cl2, 125 MHz):
δ 170.51, 168.18, 137.45, 116.02, 33.85, 29.84, 21.05.
(bpy)Pt(η2–AcN–NC(O)(CH2)2CHCH2) (4). Pt(bpy)(CH3)2 (92.0
mg, 241 mmol) and 3 (83 mg, 530 mmol) were combined in acetone
(1 mL) in a sealed tube and heated at 80 °C for 24 h. The resulting
yellow precipitate was collected and washed with acetone (0.5 mL)
and ether (1 mL) and dried to give 27.7 mg of a yellow solid (54.8
1
mmol, 23% yield). H NMR (DMF-d7, 300 MHz): δ 9.98 (d, J )
3.3, 1H, bpy H6), 8.92 (d, J ) 3.3, 1H, bpy H6′), 8.59 (d, J ) 4.5,
2H, bpy H3 and H3′), 8.38 (t, J ) 5.1, 1H, bpy H4′), 8.33 (t, J )
4.5, 1H, bpy H4), 7.73 (t, J ) 3.9, 1H, bpy H5), 7.66 (t, J ) 3.6,
1H, bpy H5′), 3.49 (m, 1H, pyr CH), 2.59 (dd, J ) 6.3, 4.2, 1H,
PtCH2), 2.45 (m, 2H, Pt-CH2, and PtCH2CHCH2), 2.32 (m, 1H,
PtCH2CHCH2CH2), 2.18 (m, 1H, PtCH2CHCH2CH2), 1.97 (s, 3H,
CH3), 1.78 (m, 1H, PtCH2CHCH2). 13C{1H} NMR (DMF-d7, 125
MHz): δ 178.28 (CO), 172.60 (CO), 155.21 (bpy C6), 151.48 (bpy
C6′), 139.90 (bpy C4 or C4′), 139.49 (bpy C4 or C4′), 128.79 (bpy
C5′), 128.22 (bpy C5), 125.16 (bpy C3 or C3′), 123.57 (bpy C3 or
C3′), 60.93 (PtCH2CH), 31.32 (Pt-CH2CHCH2CH2), 26.20
(Pt-CH2), 24.90 (Pt-CH2CHCH2), 22.20 (C(O)H3). ESI/MS
(PEt3)2Cl(H)Ir(µ-C6H4NdNC6H4)IrCl(H)(PEt3)2 (2). The dark
solids obtained above were recrystallized from THF/pentane at -78
°C to give 2 as a dark purple microcrystalline powder (17.6 mg,
0.032 mmol, 20%). 1H NMR (THF-d8, 300 MHz): δ 10.37 (d, J )
8.4, 2H, H3), 7.73 (d, J ) 7.5, 2H, H6), 6.93 (t, 7.5, 2H, H4), 6.74
(t, J ) 6.9, 2H, H5), -17.64 (t, J ) 18, 2H, IrH). 31P{1H} NMR
(THF-d8, 121 MHz): δ -4.37 (s). 13C{1H} NMR (THF-d8): δ
166.12 (C2), 152.85 (C1) 141.25 (C6), 133.54 (C3), 131.36 (C5),
121.33 (C4) 17.35 (t, J ) 17, PCH2), 8.25 (CH3). ESI/MS (CH3CN):
m/z 955 ([Ir2(PEt3)3(PhNdNPh)Cl]+). Anal. Calcd for C36H70Cl2Ir2-
N2P4: C, 38.95; H, 6.36; N, 2.52. Found: C, 39.30; H, 6.36; N,
2.46.
(CH3CN): m/z 506 [M
C17H20N4O3Pt: C, 39.01; H, 3.85; N, 10.70. Found: C, 39.27; H,
3.46; N, 10.77.
+
H]+. Anal. Calcd for 4 · H2O
Acknowledgment. We are grateful to the University of
Washington and the National Science Foundation (Grant No.
CHE-0513023 to J.M.M.) for support of this work and
Professor B. Scott Williams for helpful discussion.
Absolute assignments (6 position vs 3 position) were made by
analogy to literature.34
Pent-4-enoic Acid N′-acetyl Hydrazide (3). Pent-4-enoyl
chloride (∼14.7 mmol) was added dropwise to a solution of acyl
hydrazine (from H2NNH2 and EtOAc)78 (1.04 g, 14.0 mmol) and
Na2CO3 (11 g, 104 mmol) in CHCl3 (20 mL) at 0 °C. The reaction
mixture was stirred for 15 min, then warmed slowly to RT for an
Supporting Information Available: Figures showing the growth
of products in the reactions that yield 1, 2, 1-Tol, and 2-Tol and
CIFs for 2 and 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
(78) Khan, K. M.; Rasheed, M.; Ullah, Z.; Hayat, S.; Kaukab, F.;
Choudhary, M. I.; Rahman, A.-u.; Perveen, S. Bioorg. Med. Chem. 2003,
11, 1381–1387.
OM701192S