Facile C-N cleavage reactions of secondary and tertiary alkyl amines by P,O chelating rhodium and iridium complexes

Article abstract of DOI:10.1016/S1387-7003(02)00730-X

The reactions of Cp*MCl(MDMPP-P,O) (1: M = Rh; 3: M = Ir; MDMPP-P, O = PPh₂(C₆H₃-2-O-6-MeO)) with secondary or tertiary alkyl amine (R₂NH or R₃N) in the presence of KPF₆ underwent a C-N cleavage of amine to afford primary amine complex [Cp*M(MDMPP-P,O) (RNH₂)](PF₆) (2: M = Rh; 4: M = Ir).

Full text of DOI:10.1016/S1387-7003(02)00730-X

Inorganic Chemistry Communications 6 (2003) 202–205
www.elsevier.com/locate/inoche
Facile C–N cleavage reactions of secondary and tertiary alkyl
amines by P,O chelating rhodium and iridium complexes
*
Yasuhiro Yamamoto , Jouji Seta, Hiroshi Murooka, Xiao-Hong Han
Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi, Chiba 274-8510, Japan
Received 20 August 2002; accepted 7 November 2002
Abstract
The reactions of Cp*MCl(MDMPP-P,O) (1: M ¼ Rh; 3: M ¼ Ir; MDMPP-P; O ¼ PPh2(C6H3-2-O-6-MeO)) with secondary or
tertiary alkyl amine (R2NH or R3N) in the presence of KPF6 underwent a C–N cleavage of amine to afford primary amine complex
[Cp*M(MDMPP-P,O) ðRNH2ފðPF6Þ (2: M ¼ Rh; 4: M ¼ Ir).
Ó 2002 Elsevier Science B.V. All rights reserved.
Keywords: C–N cleavage; Rhodium; Iridium; P,O chelating ligand
Transition-metal-assisted C–X (X ¼ S, N) bond
cleavage [1] plays an important role for several indus-
trial processes. Desulfurization [2] and denitrogenation
[3] of crude oil are the examples of the C–X bond
cleavage. Among them, there are a few examples of
cleavage of a C–N single bond of amines. For example,
it has been reported that Pt, Ni and Ru complexes
promote the cleavage of the allyl-N bond of allylamines
with subsequent transfer of the allyl group to the metal
center [4]. Aryl amines ðXC6H4NH2Þ were activated by
on treatment with 1-alkyne or disubstituted alkyne in
the presence of KPF6 or NaPF6 [7]. When complexes
Cp*MCl(MDMPP-P,O) and Cp*Ir(TDMPP-P,O,O0)
(TDMPP-P,O,O0 ¼ P{C6H3-2,6-ðMeOÞ2}(C6H3-2-O-6-
MeOÞ2) were treated with electron-deficient olefins such
as tetracyanoethylene (tcne) and 7,7,8,8-tetracyano-p-
quinodimethane (tcnq), olefin inserted into weakly ac-
tivated C–H bond on the phenyl ring of the phosphine
ligand [8]. During a series of our research on interaction
of these complexes with small molecule, we found the C–
N bond cleavage reaction of tertiary and secondary
amines to give primary amine complexes. We here re-
port the facile metal-assisted C–N bond cleavage reac-
tions of alkyl amines.
t
ðsiloxÞ3Ta ðsilox ¼ Bu3SiOÞ to afford ðsiloxÞ3HTa
fNHðC6H4XÞg and/or ðsiloxÞ3ðH2NÞTaðC6H4XÞ de-
pending on the substituent [5].
Recently we reported that compounds, Cp*MCl
(MDMPP-P,O) and Cp*MCl(BDMPP-P,O) (M ¼ Rh
[a] and Ir [b]; Cpà ¼ C5Me5; MDMPP-P; O ¼
PPh2(C6H3-2-O-6-MeO), BDMPP-P,O ¼ PPh{C6H3-
2,6-ðMeOÞ2}(C6H3-2-O-6-MeO)), derived from deme-
thylation of one of the ortho-methoxy groups in
(2,6-dimethoxyphenyl)diphenylphosphine and bis(2,6-
dimethoxyphenyl)phenylphosphine gave novel six- or
seven-membered metallacycles arising from single or
double insertion of alkyne into the M–O or P–C bond
When Cp*RhCl(MDMPP-P,O) 1 was treated with
diethylamine in the presence of KPF6 at room temper-
ature, orange complex formulated as [Cp*Rh(NH2Et)
(MDMPP-P,O)](PF6) 2a based on elementary analysis
and FAB mass spectrometry, was generated in high
yield (Scheme 1). The IR spectrum showed two bands at
3323 and 3273 cmÀ1 due to N–H groups and a band at
1
885 cmÀ1 due to a PF6 group. The H NMR spectrum
showed four characteristic resonances at d 0.69(t),
1.48(d), 2.09(br), and 3.35(s), due to methyl, Cp*,
methylene and methoxy protons, respectively. The
1
31Pf Hg NMR spectrum showed a doublet at d 50.3
* Corresponding author. Tel.: +81-474-72-5076; fax: +81-474-75-
1855.
E-mail address: yamamoto@chem.sci.toho-u.ac.jp (Y. Yamamoto).
together with a septet at d-143.7 due to a PF6 group.
This complex 2a can be prepared by the reaction of
1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S1387-7003(02)00730-X

Y. Yamamoto et al. / Inorganic Chemistry Communications 6 (2003) 202–205
203
Scheme 2. Reactions of Cp*MCl ½PPh2fC6H3-2; 6-ðMeOÞ2gŠ (M ¼ Rh
or Ir) with diethyl amine.
Scheme 1. Reactions of Cp*MCl(MDMPP-P,O) (1: M ¼ Rh; 3: M ¼
Ir) with alkyl amine.
The reactions of 1 or 3 with triethylamine also gave
2a and 4a, respectively, whereas tertiary amines such
as n Pr3 N and nBu3N did not undergo a C–N bond
cleavage. No C–N cleavage reaction occurred in the
monodentate complexes such as Cp* MCl2ðLÞ
(M ¼ Rh, Ir; L ¼ PPh3; PPh2fC6H3-2; 6-ðMeOÞ2g); es-
pecially in the latter complex, an ether-O coordinated
complex [Cp*MCl{PPh2(C6H3-2-OMe-6-MeO) }](PF6)
(M ¼ Rh, Ir) [6] was isolated, suggesting the
unique specificity of the P,O chelate complexes
(Scheme 2).
Cp*RhCl(MDMPP-P,O) with ethylamine in the pres-
ence of KPF6. These results showed that the C–N bond
cleavage occurred in secondary amine, whereas did not
proceed in the primary amine. A similar C–N bond
cleavage occurred in an iridium (III) analog 3 to gen-
erate the corresponding [Cp*Ir(NH2Et)(MDMPP-
P,O)](PF6) complex 4a. It was confirmed by X-ray
analysis of 4a that ethylamine coordinated to the irid-
ium metal (Fig. 1) [9].
In order to examine a fate of fragments for amines,
the 1H NMR spectrum of a mixture of Cp*RhCl
(MDMPP-P,O), i-Pr2 NH and KPF6 was monitored in a
mixture of CDCl3 and ðCD3Þ2CO, and showed a singlet
at d 1.04 together with resonance corresponding to 2c,
suggesting the presence of 2,3-dimethylbut-2-ene, fol-
lowed by identification of 2,3-dibromo-2, 3-dim-
ethylbutane derived from its bromination. A similar
reaction using n-Bu2NH was carried out, and the GC-
MS spectrum of the reaction mixture showed the pres-
ence of octene.
When secondary amines such as di-n-propylamine,
di-iso-propylamine and di-n-butylamine were used,
the corresponding primary alkyl amine complexes,
[Cp*M(NH2R)(MDMPP-P,O)](PF6) (M ¼ Rh; 2b: R ¼
i
n Pr; 2c: R ¼ Pr; 2d: R ¼ nBu; M ¼ Ir; 4b: R ¼ n Pr; 4d:
R ¼ nBu) were generated. Secondary aromatic amine
such as Ph2NH did not undergo a C–N bond cleavage,
suggesting the requirement for the presence of the
NCH2 or NCH groups in amine molecules.
A tentative C–N cleavage route for secondary or
tertiary amines is shown in Scheme 3. Since no g1-
complexes led to the C–N cleavage, the P,rO-coordi-
nation is assumed to be indispensable. The reaction
consists of an initial coordination of alkyl amine to a
metal, accompanying with the transfer of a-hydrogen
of alkyl amine to the phenolate, forming a metalla-
aziridine. The resulting carbene-imine species by the
C–N bond cleavage rearranges to a primary amine
complex and olefin arising from a dimerization of
carbene.
The present reactions provide a mild C–N cleavage of
alkyl amines by P,O chelating complexes of pentam-
ethylcyclopentadienylrhodium(III) and -iridium(III).
We are currently investigating the catalytic application
of the reaction.
Satisfactory analytical data were obtained for new
complexes. The representative reaction is described:
Preparation of 2a: A solution of (54 mg, 0.093 mmol),
diethyl amine (0.5 ml) and NaPF6 (54.1 mg, 0.322
mmol) was stirred in CH2Cl2 (7.5 ml) and acetone (7.5
ml) at rt. After 4 h, the solvent was removed, and residue
was extracted with CH2Cl2. The CH2Cl2 was concen-
trated, and diethyl ether was added to give orange
Fig. 1. Molecular structure of complex cation [Cp*IrCl(MDMPP-P,O)
ðEtNH2ފðPF6Þ 4a. The anion was omitted for clarity. Selected bond
ꢀ
length (A) and angles (°): Ir–P(1) 2.304(3), Ir–O(1) 2.074(7), Ir–N
2.158(8); P(1)–Ir–O(1) 82.3(2), P(1)–Ir–N(1) 87.9(2), O(1)–Ir–N(1)
85.1(3).

204
Y. Yamamoto et al. / Inorganic Chemistry Communications 6 (2003) 202–205
Scheme 3. A possible pathway for the C–N cleavage reaction. The PF6 anion was omitted for clarity.
1
brown crystals of 2a (55.2 mg, 80.8%). IR(nujol): 3325,
3285(N–H), 839(PF6) cmÀ1; 1H NMR(CDCl3): d 0.69 (t,
JHH ¼ 7:0 Hz, Me, 3H), 1.48 (d, JPH ¼ 3:0 Hz, Cp*,
15H), 2.09 (br, CH2, 2H), 3.35 (s, OMe, 3H), 3.40 (b,
Monitoring of the reaction mixture by the H NMR
spectrum: 2,3-dimethylbut-2-ene was estimated as ca.
55% yields based on the intensity of the methyl reso-
nance.
1
NH2, 2H), 6.0–7.6 (m, 13H). 31Pf Hg NMRðCDCl3): d
50.3 (d, JPRh ¼ 137 Hz), )143.7 (sep, JPF ¼ 712 Hz).
(Yield of 2a from Et3N: 61.6%). 2b (80%, from
References
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;
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0.95 (m, CH2, 2H), 1.48 (d, JPH ¼ 3:0 Hz, Cp*, 15H),
1.99 (br, CH2, 2H), 3.35 (s, OMe, 3H), 6.0–7.6 (m, 13H).
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1
31Pf Hg NMRðCDCl3Þ: d 50.0 (d, JPRh ¼ 125 Hz),
)143.8 (sep, JPF ¼ 713 Hz). 2c (%, from i Pr2 NH): FAB
mass (m=z): 606 (½MŠþ). IR(nujol): 3314, 3274 (NH), 839
(PF6) cmÀ1; 1H NMRðCDCl3Þ: d 0.36 (d, JHH ¼ 6:5 Hz,
i Pr, 3H), 0.95 (d, JHH ¼ 6:5 Hz, i Pr, 3H), 1.47 (d,
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1
OMe, 3H), 6.0–7.7 (m, 13H). 31Pf Hg NMRðCDCl3Þ: d
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(orange, 48.3% from nBu2NH): IR(nujol): 3319, 3277
(NH), 835 (PF6) cmÀ1
;
1H NMRðCDCl3Þ: d 0.62 (t,
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JHH ¼ 7:0 Hz, Me, 3H), 0.85 (b, CH2, 6H), 1.48 (d,
JPH ¼ 3:0 Hz, Cp*, 15H), 2.02 (b, CH2), 2.35 (b, NH2,
(c) T.C. Ho, Catal. Rev.-Sci. Eng. 20 (1979) 155;
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1
NMRðCDCl3Þ: d 49.9 (d, JPRh ¼ 139 Hz), )143.8 (sep,
JPF ¼ 714 Hz). 4a (yellow, 16.2% from Et3N; 57.2%
from Et2NH): FAB mass (m=z): 694 ð½MŠþÞ; IR(nujol):
3312, 3265(N–H), 844 (PF6) cmÀ1; 1H NMRðCDCl3Þ: d
0.72 (t, JHH ¼ 7:0 Hz, Me, 3H), 1.52 (d, JPH ¼ 2:0 Hz,
Cp*, 15H), 2.16 (q, JHH ¼ 7:0 Hz, CH2, 2H), 3.36 (s,
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1
26.3 (s), )143.7 (sep, JPF ¼ 712 Hz). 4b (yellow, 71.1%,
from n Pr2 NH): IR(nujol): 3302, 3260 (NH), 839 (PF6)
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1
cmÀ1; H NMRðCDCl3Þ: d 0.43 (t, JHH ¼ 7:5 Hz, Me,
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1
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(NH), 837 (PF6) cmÀ1
;
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1
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NMRðCDCl3Þ: d 26.2 (s), )143.8 (sep, JPF ¼ 711 Hz).

Y. Yamamoto et al. / Inorganic Chemistry Communications 6 (2003) 202–205
205
[9] Crystal data for 4a: C31H38NOP2F6Ir, M ¼ 808:8, monoclinic,
space group P21=c (No. 14), Z ¼ 4, a ¼ 12:541ð2Þ, b ¼
and refined anisotropically using full-matrix least-square tech-
niques based on F 2 for all unique reflections (5914) to give
R ¼ 0:117 and Rw ¼ 0:149 ½w ¼ 1=r2ðFoފ for 5914 reflec-
tions, and R1 ¼ 0:056 (for 4134 reflections I > 2:0rðIoÞ).
GOF ¼ 1.48.
ꢀ
ꢀ3
14:708ð4Þ, c ¼ 17:539ð3Þ A, b ¼ 92:19ð1Þ°, V ¼ 3232:9ð9Þ A ,
D ¼ 1:662 g=cm3. Data were collected on a Rigaku AFC5S
diffractometer. The structure was solved by Patterson methods