Neighbouring metal induced oxidative addition at the iron centre amongst the iron–arylpyridylphosphine complexes

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Neighbouring metal induced oxidative addition at the iron centre amongst the

iron–arylpyridylphosphine complexes

Olalere G. Adeyemi

a,b,*

, Ling-Kang Liu

a,c a

Department of Chemical Sciences, College of Natural Sciences, Redeemer’s University, KM 46, Lagos-Ibadan Expressway, Redemption City, Nigeria

b_{Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan, ROC} c_{Department of Chemistry, National Taiwan University, Taipei 10767, Taiwan, ROC}

a r t i c l e

i n f o

Article history: Received 28 March 2008

Received in revised form 24 April 2008 Accepted 24 April 2008

Available online 6 May 2008 Keywords: Oxidative addition Hydrides Iron complexes Arylpyridylphosphines

a b s t r a c t

Complexes of the type (g4_-BuC

5H5)Fe(CO)2(P) (P = PPh2Py 3, PPhPy24, PPy35; Py = 2-pyridyl) were

sat-isfactorily prepared. Upon treatment of 3 with M(CO)3(EtCN)3(M = Mo, 6a; W, 6b), the pyridyl N-atom

could be coordinated to the metal M, which then eliminates a CO ligand from the Fe-centre and induced an oxidative addition of the endo-C–H of (g4_-BuC

5H5). This results in a bridged hydrido heterodimetallic

complex [(g5_-BuC

5H4)Fe(CO)(l-P,N-PPh2Py)(l-H)M(CO)4] (M = Mo, 7a, 81%; W, 7b, 76%). The reaction of

4 or 5 with 6a,b did not give the induced oxidative addition, although these complexes contain more than one pyridyl N-atom. The reaction of 4 with M(CO)4(EtCN)2(M = Mo, 9a; W, 9b) produced

heterodimetal-lic complexes [(g4_-BuC

5H5)Fe(CO)2(l-P:N,N0-PPhPy2)M(CO)4] (M = Mo, 10a, 81%; W, 10b, 83%).

Treat-ment of 5 with 6a,b gave [(g4_-BuC

5H5)Fe(CO)2(l-P:N,N0,N00-PPy3)M(CO)3] (M = Mo, 12a, 96%; W, 12b,

78%).

1. Introduction

The coordination chemistry of arylpyridylphosphines PPh3nPyn

(n = 1–3; Py = 2-pyridyl) has largely developed. It is of particular interest because of the use of metal complexes containing such li-gands in homogeneous catalysis, especially with a potential appli-cation in aqueous media[1]. The presence of a basic N-atom in each pendant pyridine in PPh3nPynligands distinguishes it from

PPh3 ligand. Hence, there is enhanced acid–base chemistry

involved by the presence of pyridyl N-atom(s) and more coordina-tion modes are available for PPh3nPyn as ligand than only the

P-donor mode in PPh3. For instance, PPh2Py could behave in

mono-nuclear complexes as a P-donor ligand and in dimono-nuclear complexes as a P,N-bridging ligand with or without metal–metal bonding. PPhPy2and PPy3 can bind further with a variety of coordination

modes: through two pyridyl groups (N,N0_{; PPh}

2Py and PPy3),

through three pyridyl groups (N,N0_,N00_PPy

3), and via the

phospho-rus and two pyridyl groups (P,N,N0_{; PPhPy}

2and PPy3)[2].

Few years ago we reported the synthesis of (

g

4

-BuC5H5

)Fe-(CO)2(PPh3nPyn) (n = 1–3). A three-component reaction was

uti-lized to produce the complexes. The hydride abstraction of

endo-hydrogen of (

g

4_-BuC

5H5)Fe(CO)2(PPh3nPyn) were smoothly

car-ried out by Lewis acid, HBF4 OEt2 to give [(

g

5-BuC5H4

)Fe-(CO)2(PPh3nPyn)]+[BF4](n = 1–2)[3].

In this paper, we incorporated the PPh3nPynligands into

com-plexes of the type (

g

4_-BuC

5H5)Fe(CO)2(PPh3nPyn) and studied the

effect of the presence of the dangling pyridyl N-atom(s), known to be able to coordinate to a second metal centre[2]. In the case of the PPh2Py complex which contains one pyridyl N-atom, a

neighbour-ing Mo or W metal originally with three labile ligands could induce an oxidative addition on the Fe-centre. Similar chemistry could not be observed for the PPhPy2and PPy3analogues.

2. Experimental 2.1. General

All manipulations were performed under an atmosphere of pre-puriﬁed nitrogen with standard Schlenk techniques and a double-fold vacuum line. All solvents were distilled from an appropriate drying agent[4]. For instance, THF, Et2O and n-hexane were dried

over sodium benzophenone ketal. CH3CN and CH3CH2CN were

dried over P2O5and CaH2was used for CH2Cl2. Infrared spectra

were recorded in CH2Cl2 using CaF2 optics on a Perkin–Elmer

(FT-IR) Paragon 1000 spectrophotometer. The 1_{H NMR and} 13_C

NMR spectra were obtained on Bruker AC200/AC300 spectrome-ters, with chemical shifts reported in d values, downﬁeld positive, relative to the residual solvent resonance of CDCl3(1H d 7.24,13C

* Corresponding author. Address: Department of Chemical Sciences, College of Natural Sciences, Redeemer’s University, KM 46, Lagos-Ibadan Expressway, Redemption City, Nigeria. Tel.: +234 805 5516450.

E-mail addresses: drlereadeyemi@yahoo.com, adeyemio@run.edu.ng (O.G. Adeyemi).

Contents lists available atScienceDirect

Inorganica Chimica Acta

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i c a

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NMR d 77.0). The31_{P NMR spectra were obtained on Bruker AC200/}

AC300 spectrometers using 85% H3PO4as an external standard (d

0.00). The melting points were determined on a Yanaco MPL melt-ing-point apparatus and are uncorrected. Mass spectra were re-corded on a VG 70 – 250S mass Spectrometer, using fast atomic bombardment technique, independently operated by the Institute of Chemistry, Academia Sinica. Chemical analysis was performed on a Perkin–Elmer 2400 CHN elemental analyzer, also indepen-dently operated by the Institute of Chemistry, Academia Sinica. Compounds 1,[5], 2, 3, 4, 5[3], 6a,b,[6]and 9a,b[7]were prepared according to literature procedure. Ligands PPh2Py [8a], PPhPy2 [8b], PPy3 [8c] were prepared using Li–Br exchange method in

good yield. PC13, PPhC12, PPh2C1, and o-C5H4NBr were obtained

from commercial sources, distilled twice and degassed prior to use. Other reagents were obtained from commercial sources and used without further puriﬁcation.

2.2. [(

g

5_-BuC

5H4)Fe(CO)(

l

-P:N-PPh2Py)(

l

-H)Mo(CO)4] 7a

Compound 3 (0.30 g, 0.60 mmol) reacted with 6a (0.30 g, 0.87 mmol) and were dissolved in dry THF (30 mL). The mixture was warmed up to ensure complete dissolution, to give orange solution. The mixture was then stirred at room temperature for 2 h to give a deep orange solution which was ﬁltered via a pad of dry celite under nitrogen and washed with dry ether until col-ourless to give clear orange solution. The solvent was removed un-der vacuum. The ensuing orange solid was dissolved in a little quantity of CH2C12and mixed well with a little quantity of Al2O3

before being pack on a short Al2O3column. The orange band was

eluted with 1:8–1:5 EtOAc/n-hexane mixtures. Bright orange powder of 7a was collected after removing the solvent under vacuum.

Compound 7a: yield: 0.33 g, 81%; mp: 111 °C (dec.); IR (CH2Cl2)

m

CO2016 (m), 1927 (vs), 1896 (s), 1879 (sh), 1830 (s) cm1;31P

NMR (CDCl3) d 86.12 (s); 1H NMR (CDCl3) d 9.25 (b, 1H, Py),

6.93–7.68 (m, 13H, Ph and Py), 4.75, 4.66, 4.41, 4.30 (4 b, 4 1H, Cp), 0.87–2.42 (m, 9H, Bu), 16.46 (d, 1H,

l

-H,

2_J

PH= 56 Hz); MS m/z 678 (M++1). Anal. Calc. for C31H28FeMoNO5P:

C, 54.95; H, 4.14; N, 2.07. Found: C, 55.24; H, 4.85; N 1.77%. 2.3. [(

g

5_-BuC

5H4)Fe(CO)(

l

-P:N-PPh2Py)(

l

-H)W(CO)4] 7b

Compound 3 (0.30 g, 0.60 mmol) and 6b (0.30 g, 0.69 mmol) were dissolved in dry THF (30 mL). The procedure is similar to that of the reaction of the molybdenum analogue except that the puri-ﬁcation was carried out using a silica gel column. The orange solid was dissolved in a small quantity of CH2C12and mixed well with a

small quantity of silica gel before being packed on top of a silica gel column, and then eluted with a EtOAc/n-hexane mixture. The or-ange powder of 7b was collected after solvent removal under vacuum.

Compound 7b: yield: 0.35 g, 76%, mp: 120 °C (dec.); IR (CH2C12)

m

CO 2009 (m), 1927 (s) 1886 (vs, b), 1827 (s) cm1; 31P NMR (CDC13) d 88.6 (s);1H NMR (CDC13) d 9.38 (b, 1H, Py), 6.90–7.48 (m, 13H, Ph and Py), 4.77, 4.72, 4.51, 4.31 (4 b, 4 1H, Cp), 0.87–2.63 (m, 9H, Bu), 14.21 (d, 1H,

l

-H,2_J PH= 54 Hz); MS m/z 766 (M+_{+1). Anal. Calc. C} 31H28FeNO5PW: C, 48.63; H, 3.66; N, 1.83. Found: C, 48.78; H, 3.98; N, 1.75%. 2.4. [(

g

4_-BuC

5H5)Fe(CO)2(

l

-P:N,N0-PPhPy2)Mo(CO)4] 10a

Compound 4 (0.10 g, 0.20 mmol) and 9a (0.10 g, 0.31 mmol) were completely dissolved in dry THF (10 mL). The colour changed to blood red immediately. The mixture was allowed to stir for 1.5 h before ﬁltration over a bed of dry celite under nitrogen. The solvent was removed under vacuum and the red solid re-crystallized twice

with ether/n-hexane mixture (1:1) and ﬁltered to give a brick red solid of 10a. Using (C7H8)Mo(CO)4(80 mg, 0.27 mmol, C7H8=

bicy-clo [2,2] hepta-2,5-diene) to replace 9a, the result is the same with very similar yields.

Compound 10a: yield: 0.114 g, 81%, mp: 138 °C (dec.); IR (CH2C12)

m

CO 1977 (s), 1919 (vs) 1816 (s, b) cm1; 31P NMR (CDC13) d 95.5 (s); 1H NMR (CDC13) d 9.24 (b, 2H, Py), 4_J PH= 4.95 Hz, 6.53–8.30 (m, 11H, Ph and Py), 4.95 (b, 2H, –CH@CHCHBu–), 2.49 (b, 1H, –CH@CHCHBu–), 2.22 (b, 2H, –CH_{@CHCHBu–), 0.51–1.08 (m, 9H, Bu), MS (m/z) 497} (M+

Mo(CO)4+ 1), 441 (M+Mo(CO)6+1). Anal. Calc. C31H27

FeMo-N2O6P: C, 52.69; H, 3.82; N, 3.97. Found: C, 52.16; H, 4.43; N,

3.65%. 2.5. [(

g

4_-BuC

5H5)Fe(CO)2(

l

-P:N,N0-PPhPy2)W(CO)4] 10b

The procedure is similar to that of the reaction of the molybde-num analogue with 4 (0.30 g, 0.60 mmol) and 9b (0.30 g, 0.74 mmol) in dry THF (10 mL). Dark purple solid of 10b was col-lected after the necessary work-up.

Compound 10b : yield: 0.40 g, 83%; mp: 143 °C (dec.); IR (CH2C12)

m

CO 1977 (s), 1912 (vs) 1813 (s, b) cm1; 31P NMR

(CDC13) d 97.9 (s);1H NMR (CDC13) d 9.30 (b, 2H, Py), 6.40–8.28

(m, 11H, Ph and Py), 4.98 (b, 2H, –CH@CHCHBu–), 2.52 (b, 1H, – CH@CHCHBu–), 2.29 (b, 2H, –CH@CHCHBu–), 0.56–1.06 (m, 9H, Bu), MS m/z 767 (M+CO+1) 497, (M+W(CO)4+1). Anal. Calc. for

C31H27FeN2O6PW: C, 46.85; H, 3.40; N, 3.53. Found: C, 46.32; H,

3.66; N, 3.33%. 2.6. [(

g

4_-BuC

5H5)Fe(CO)2(

l

-P:N,N0N00-PPy3)Mo(CO)3] 12a

Compound 5 (0.20 g, 0.40 mmol) and 6a (0.20 g, 0.58 mmol) were completely dissolved in dry THF (20 mL). The mixture turned purple immediately. The reaction was left for 1 h before being ﬁl-tered under nitrogen through a pad of dry celite and washed with dry ether until colourless. The solvent was removed under vacuum, resulting in a dark purple solid, which was dissolved in a small quantity of CH2C12and mixed completely with a small quantity

of alumina. The CH2C12was removed under vacuum. The residue

was packed on top of an alumina column and then eluted with a 3:1 EtOAc/n-hexane mixture. A deep purple solid 12a was collected.

Compound 12a: yield: 0.26 g, 96%; mp: 158 °C (dec.); IR (CH2C12)

m

CO 1981 (m), 1924 (m), 1905 (vs), 1788 (s, b) cm1; 31_{P NMR (CDC1}

3) d 75.4 (s);1H NMR (CDC13) d 9.47 (b, 3H), 8.48

(b, 3H), 7.85 (b, 3H), 7.20 (b, 3H) for pyridine, 5.35 (b, 2H, – CH@CHCHBu–), 3.27 (b, 2H, –CH@CHCHBu–), 3.04 (b, 1H, – CH@CHCHBu–), 0.83–1.15 (m, 9H, Bu), MS m/z 498 (M+_Mo(CO)

3).

Anal. Calc. C29H25FeMoN3O5P: C, 51.33; H, 3.69; N, 6.19. Found: C,

50.38; H, 3.95; N, 5.73%. 2.7. [(

g

4

-BuC5H5)Fe(CO)2(

l

-P:N,N0N00-PPy3)W(CO)3] 12b

Compound 5 (0.20 g, 0.40 mmol) and 6b (0.20 g, 0.46 mmol) were dissolved in dry THF (10 mL). Again the mixture immediately changed to purple colour. The reaction was then left for 2 h. Other procedure and work-up were similar to those used in the molybde-num analogue.

Compound 12b: yield: 0.24 g, 78%; mp: 165 °C (dec.); IR (CH2C12)

m

CO1982 (m), 1926 (s), 1895 (vs), 1782 (s, b) cm1;31P

NMR (CDC13) d 77.2 (s); 1H NMR (CDC13) d 9.48 (b, 3H) 8.50

(b, 3H), 7.88 (b, 3H), 7.15 (b, 3H) for pyridine, 5.38 (b, 2H, –CH@CHCHBu–), 3.31 (b, 2H, –CH@CHCHBu–), 3.06 (b, 1H, –CH@CHCHBu–), 0.82–1.24 (m, 9H, Bu); MS m/z 498 (M+_W(CO)

3).

Anal. Calc. for C29H25FeN3O5PW: C, 45.43; H, 3.26; N, 5.43. Found:

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3. Results and discussion

3.1. Preparation of arylpyridylphosphine ligands

The preparation of PPh3nPyn(n = 1–3) followed the literature

procedures but with a slight local modiﬁcation[8a,8b,8c]. In our own experience, the preparation of PPhPy2gave a yield of 19.7%

when the Grignard reagent on the pyridyl side is used for coupling with PPhCl2. The preparation of PPhPy2gave a much better yield of

54.5% by using the Li–Br exchange reaction between n-BuLi and o-C5H4NBr, then the coupling reaction with PPhC12. The preparation

of PPh3nPyn (n = 1–3) in the present study involved the n-BuLi

route and the yields were satisfactory (PPh2Py, 82%; PPhPy2, 55%;

PPy3, 62%). Pure PPy3was stored in the dry box in order to avoid

oxidation of the phosphine to Py3P@O.

3.2. (

g

4

-BuC5H5)Fe(CO)2(PPh3nPyn) complexes

The arylpyridylphosphine ligands were reacted with a stoichi-ometric amount of (

g

5

-C5H5)Fe(CO)2I (1) and a slight excess

amount of n-BuLi, using the same conditions as those used in the preparation of (

g

4_-BuC

5H5)Fe(CO)2(PPh3) (2) [3]. With one basic

N-atom in each pendant pyridine, the arylpyridylphosphines re-acted smoothly in the three-component reaction and gave simi-larly (

g

4_-BuC

5H5)Fe(CO)2(PPh3nPyn) in good yields of relevant

complexes (3, n = 1, 68%; 4, n = 2, 72%; 5, n = 3, 74%). The chemistry followed the same pathway as in the PPh3case[3].

Scheme 1shows the two stages in the three-component prepa-ration of compounds 2–5. The ﬁrst drop of n-BuLi acted as a reduc-ing agent[9]to initiate an electron-transfer chain catalysis[10]of the replacement of iodide on 1 by PPh3nPyn with the P-donor

mode. The cation [(

g

5_-C

5H5)Fe(CO)2(PPh3nPyn)+] (n = 0–4) is

instantaneously obtained in this reaction[11]. The stoichiometric n-BuLi then acts as a normal nucleophile to add on to the (

g

5

-C5H5)-ring [12,13]. The electron rich (

g

5-C5H5)-ring of neutral 1

did not react with the electron rich [Bu_{] anion in an ordinary}

way. In its cationic form, the (

g

5_-C

5H5)-ring is more likely

acti-vated as a Lewis-acid. [(

g

5

-C5H5)Fe(CO)2(PPh3nPyn)+] is much

more electrophilic towards [Bu_{] and the addition of [Bu}_{] anion}

on the (

g

5_-C

5H5)-ring is favoured[3].

3.3. Heterodimetallic complexes

In the main synthetic routes to organo-transition metal hy-drides, the intramolecular oxidative addition is a key to the facile loss of hydrogen, for example, in (

g

4_-C

5H6)Fe(CO)3which proceeds

with an intermediate formation of (

g

5_-C

5H5)Fe(CO)2H, to ﬁnally

yield (

g

5_-C

5H5)Fe(CO)2]2[14]. Yet, it is possible to independently

synthesize (

g

5_-C

5H5)Fe(CO)2H from the acidiﬁcation of (

g

5

-C5H5)Fe(CO)2Na. In the literature, it was not synthesized directly

Fe OC CO I + PR3 cat n-BuLi Fe OC CO PR3 + I -n-BuLi -78oC to RT -LiI Bu H Fe OC CO PR3 PR3 PPh3 PPh2Py PPhPy2 PPy3 2 3 4 5 (83%) (68%) (72%) (74%) -78oC 1 Scheme 1. Bu H Fe OC CO P N Ph2 M L CO L CO L CO + -3L Fe OC Ph2P Bu N M H CO OC CO CO -3L Bu H Fe OC Ph2P N M CO CO CO CO OC Fe Ph2P N M CO CO CO Bu H OC Fe OC Ph2P Bu N M H CO OC CO CO M = Mo 6a W 6b L = CH3CH2CN 7a, b Scheme 2.

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from (

g

4_-C

5H6)Fe(CO)3due to competing and/or consecutive

reac-tions[15].

We discuss below a clean activation of the endo-C–H bond of 3 that was formally a (

g

4_{-cyclopentadiene)-Fe complex in PPh}

2Py

derivative. When 3 is reacted with M(CO)3(EtCN)3 (M = Mo, 6a;

W, 6b) with the labile EtCN to be replaced by the pyridyl N-atom and others, the result is the hydridobridged heterodimetallic com-plex [(

g

5_-BuC

5H4)Fe(CO)(

l

-P:N-PPh2Py)(

l

-H)M(CO)4] (M = Mo, 7a,

81%; W, 7b, 76%) with six-membered heterocyclic ring (Scheme 2). The environments around Fe and M in 7a,b changed substan-tially from the starting environments of 3 and 6a,b as shown by the complete disappearance of all the original IR

m

CObands: the

1966 (s) cm1_{bands for the two CO ligands connected to Fe in 3,}

the 1921 (s), 1801 (s) cm1_{for the three CO ligands connected to}

Mo in 6a, and the 1910 (s), 1793 (s) cm1_{for the three CO ligands}

connected to W in 6b. The1H NMR data also indicated that the 2:1:2 integration ratios for the (

g

4_-BuC

5H5) fragment of 3

disap-peared. Whereas a 4-peak, equal-intensity pattern characteristic of a (

g

5_-BuC

5H4) connected to a chiral metal centre appeared (cf.

at d 4.75, 4.66, 4.41, 4.30 in 7a and at d 4.77, 4.72, 4.51, 4.31 in 7b). The doublet hydrides at d 16.5 with2_J

PH= 56 Hz in 7a and

at d 14.2 with2_J

PH= 54 Hz in 7b were observed in the1H NMR

spectra. The molecular structure of 7a,b was ﬁnalized on the basis of a similar compound [(

g

4_-MeC

5H4)Fe(CO)(

l

-P:P0-PPh2CH2PPh2

)-(

l

-H)M(CO)4] (M = Mo and W)[16]that was prepared from 6a,b

and a [(

g

4

-MeC5H5)Fe(CO)2(

g

1-PPh2CH2PPh2) with the dangling

PPh2 working the same way as the present pyridyl N-atom. The

dppm analogue revealed very similar spectroscopic data in the IR and1H NMR spectra to those of 7a,b, especially those of the hy-dride and (

g

5_-MeC

5H4) ligands connected to a chiral metal centre.

Earlier, the origin of the hydride and additional CO on M-centre of [(

g

5_-MeC

5H4)Fe(CO)(

l

-P:P0-PPh2CH2PPh2)(

l

-H)M(CO)4] (M = Mo

and W)[16]was assigned as intramolecularly converted from the endo-H and the CO on Fe-centre, based on quantitative yield during synthesis. The hydride remained un-changed with the deuterium atoms of d-solvent when the reaction was performed in d8_-THF.

It is pertinent to state that, crystal suitable for X-ray crystallogra-phy for these compounds was attempted, but did not succeed.

Apparently in Scheme 2, it shows that, upon pyridyl N-atom ligation to the second metal (M = Mo, W), this neighbouring metal M induces an oxidative addition of the endo-C–H of cyclopentadi-ene on the initial Fe-centre, by taking away a CO ligand from the Fe-centre. Originally the Fe-centre in 3 is penta-coordinate. The loss of CO to M-centre results in a tetra-coordinate Fe(0), ready to activate the nearby endo C–H bond. Along the reaction coordi-nate, Fe(0) then changes to a hexa-coordinate Fe(II). At a later stage, the second metal M traps the newly formed Fe–H bond in the form of ‘‘3c–2e” M–H–Fe structure. The relative conformation with respect to Fe and M is not yet clear during the stage of the pyridyl N-atom ligation or during the stage of the CO-migration. The ﬁnal conformation with respect to Fe and M is in a syn-form in 7a,b. N M' P M Ar Ar N P M Ar Ar M' syn-form anti-form

Drawings of syn- and anti- forms

Bu H Fe OC OC P M L CO L CO L CO + N N Fe OC P Bu M H CO OC CO CO Bu H Fe OC OC P M OC CO L CO L CO + N N Bu H Fe OC OC P N N M CO CO CO CO +CO (1 atm) 4 6a, b 8a, b 4 M = Mo 9a W 9b L = CH3CH2CN 10a, b N N Scheme 3.

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With two pyridyl N-atoms, 4 gave no hydride formation when it was reacted with 6a,b. There was no spectroscopic evidence to support the structure [(

g

5

-BuC5H4)Fe(CO)(

l

-P:N-PPhPy2)(

l

-H)-M(CO)4] (M = Mo, 8a; W, 8b) (Scheme 3).

When 4 was treated with M(CO)4(EtCN)2(M = Mo, 9a; W, 9b) as

shown inScheme 3, the N,N0_{-chelated heterodimetallic complex}

was collected in high yield, i.e., [(

g

4_-BuC

5H5)Fe(CO)2(

l

-P:N,N0

-PPhPy2)M(CO)4] (M = Mo, 10a, 81%; W, 10b, 83%) (C7H8Mo(CO)4

(C7H8= bicyclo [2,2] hepta-2,5-diene) replacing 9a gave the same

result). Alternative route to the synthesis of 10a, b was achieved when CO (1 atm) was bubbled into the solution of 4 and 6a,b to produce variable yields of 10a,b.

The reaction of 5 and 6a,b shown inScheme 4, did not give com-pound [(

g

5_-BuC

5H4)Fe(CO)(

l

-P:N-PPy3)(

l

-H)M(CO)4] (M = Mo,

11a; W, 11b), but the N,N0_N00_{-coordinated heterodimetallic}

com-plex [(

g

4_-BuC

5H5)Fe(CO)2(

l

-P:N,N0,N00-PPy3)M(CO)3] (M = Mo,

12a, 96%; W, 12b, 78%).

The IR

m

COdata for the two CO ligands connected to Fe exhibit a

shift to higher wave numbers upon attachment of M(CO)3to the

three pyridyl N-sites of PPy3, i.e., from 1969, 1909 cm1 in 5 to

1981, 1924 cm1_{in 12a and 1982, 1926 cm}1_{in 12b. Upon}

attach-ment of M(CO)4to the two pyridyl N-sites of PPhPy2, similar shifts

are found in 10a,b, to 1977, 1919 cm1 _{and 1977, 1912 cm}1_,

respectively, comparing with 1967, 1907 cm1 _{in 4. It was}

ob-served that, complexes 10a,b and 12a were not excellently stable when expose to air. This could attributes to high values recorded for carbon in the elemental analysis results.

4. Conclusion

Organo-transition metal hydrides could be produced through the intramolecular oxidative addition reaction between endo-C–H

of cyclopentadiene iron with one pyridyl N-atom and M(CO)3L3

(M = Mo, W; L = labile ligand). Yet, there is no hydride formation with complexes of more than one pyridyl N-atoms e.g. (

g

4

-BuC5H5)Fe(CO)2(PPhPy2) and (

g

4-BuC5H5)Fe(CO)2(PPy3). The CO

transfer from Fe to M shown inScheme 2is more likely to be the slow step than the pyridyl N-ligation to M, to the state, ready for oxidative addition of Fe(0) in the reaction coordinate. Apparently, in 4 and 5, extra pyridyl N-atom(s) are involved to mask the migra-tion of CO ligand from Fe-centre to M-centre. Accordingly, these two compounds do not exhibit an induced activation of the endo-C–H bond of the cyclopentadiene on Fe-centre.

Acknowledgement

Thanks are due to the National Science Council and Academia Sinica for the kind ﬁnancial support.

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