Cyclopentadienyl chromium complexes with halide, methyl, isothiocyanate and isoselenocyanate ligands: Structures of [η5-(C5H4-COOCH3)]Cr(NO)2(Br) and [η5-(C5H4-COOCH3)]Cr(NO)2(N＝C＝S)

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Cyclopentadienyl chromium complexes with halide,

methyl, isothiocyanate and isoselenocyanate ligands: Structures

of [g

5

-(C

₅

H

₄

-COOCH

₃

)]Cr(NO)

₂

(Br)

and [g

5

-(C

₅

H

₄

-COOCH

₃

)]Cr(NO)

₂

(N

@C@S)

Yu-Pin Wang

a,*

, Hsien-Li Leu

a

, Yu Wang

b

, Hsiu-Yao Cheng

a

, Tso-Shen Lin

a

Department of Chemistry, Tunghai University, Taichung, Taiwan, ROC b

Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC Received 25 January 2007; received in revised form 6 March 2007; accepted 6 March 2007

Available online 7 April 2007

Abstract

Bromination/nitrosylation of [g5-(carbomethoxy)cyclopentadienyl]dicarbonylnitrosylchromium (8) (hereafter called carbo-methoxycynichrodene) with hydrogen bromide/isoamyl nitrite gives bromo [g5_{-(carboxymethoxy)cyclopentadienyl]dinitrosylchromium}

(10) in 84%. Compounds 15 in 74% and 16 in 90% were obtained from the corresponding cynichrodene derivatives via the same method. Compounds [g5-(carbomethoxy)cyclopentadienyl](isothiocyanato)dinitrosylchromium (13) and [g5 -(carbomethoxy)cyclopen-tadienyl](isoselenocyanato)dinitrosylchromium (14) were prepared from [g5 -(carbomethoxy)cyclopentadienyl]chlorodinitrosylchro-mium (9) with excess potassium thiocyanate and selenocyanate, respectively, after detaching the chloride by the action of silver nitrate. One of the nitrosyl groups in each compound is located at the site away from the exocyclic carbonyl carbon of the Cp(Cr) ring with twist angles of 168.5 and 172.3, respectively. The chemical shifts of C(2)–C(5) carbon atoms of a series of substituted-cyclopentadienyldinitrosylchromium derivatives, [g5_-(C

5H4-sub)]Cr(NO)2X, have been assigned using two-dimensional HetCOR

NMR spectroscopy. The assigned chemical shifts were compared with the NMR data of their analogues of ferrocene, and the opposite correlation on the assignments was observed. The electron density distribution in the cyclopentadienyl ring is discussed on the basis of

13

C NMR data and those of 10 and 13 are compared with the calculations via density functional B3LYP correlation-exchange method.

Keywords: Organometallics; Cyclopentadienyl; Chromium; Cynichrodene; Nitrosyl; Isothiocyanate; Isoselenocyanate; Structure;13C NMR, B3LYP

1. Introduction

Since the advent of ferrocene in the early 1950s, the syntheses and characterizations of metallocenes have been extensively studied, especially for the iron and tungsten compounds[1]. However, the number of chromium metal-locenes being studied is relatively small. The number of isolated and well characterized Cp-substituted

bromo-chromium, isothiocyanatochromium or isoselenocyanato-chromium complexes is even more limited. In the case of CpCr(NO)2Br, only one compound, Cp*Cr(NO)2Br

[2], is reported in the literature. For Cp derivatives of compounds CpCr(NO)2(N@C@S) and CpCr(NO)2

-(N@C@Se), there are none.

The Cp-chromium compounds may have properties dis-tinct from their iron analogues. Earlier[3], we reported the unequivocal assignments of C(2,5) and C(3,4) on the Cp ring of the CpCr(CO)2(NO)(cynichrodene) derivatives

bearing electron-withdrawing substituent in 13C NMR spectra. The opposite correlation on the assignments

*

Corresponding author. Fax: +88 6423590426. E-mail address:ypwang@thu.edu.tw(Y.-P. Wang).

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between ferrocene and cynichrodene (1) was a surprising finding. In the case of ferrocene [4], the 3,4-positions are more sensitive to the electron-withdrawing substituent, while in the case of cynichrodene, the 2,5-positions are more sensitive to the electron-withdrawing substituent. The overall electron-withdrawing property of CO and NO ligands may exert the difference [3a]. This amazing finding prompted us to study the Cp-derivatives of 2–7, compounds with electron-withdrawing ligands, containing (NO)2Cl, (NO)2Br, (NO)2(I), (NO)2(CH3), (NO)2

(N-C@S), and (NO)2(N@C@Se) moieties of chromium

metal-locenes[5].

Compounds 3, 6 and 7 were ﬁrst reported in 1956, 1956

[6]and 1968 [7], respectively. The diﬃculties encountered for compounds 1–7 to undergo electrophilic aromatic sub-stitution reactions such as Friedel–Crafts acylation have blocked the way to the synthesis of their respective Cp-derivatives [8]. A novel method of replacing dicarbonyl with (NO)Cl ligand with hydrogen chloride/isoamylnitrite has been revealed by us to convert 1 to 2[9]. The bromide analogues 10 and 15–16 were prepared with the use of hydrogen bromide/isoamylnitrite in place of hydrogen chloride/isomaylnitrite. The availability of 9 [9] makes the syntheses of carbomethoxy derivatives 13 and 14 accessible.

Herein, we report thorough spectral studies on 10, and 13–16, and the crystal structures of [g5-(C5H4

-COOCH3])Cr(NO)2Br (10) and [g5-(C5H4-COOCH3

)-Cr(NO)2(N@C@S) (13), the ﬁrst X-ray conﬁrmed

Cr-isothiocyanate bonding structure [10]. 13C NMR spectral comparisons between compounds 1–16 are also included. Cr R ON _COCO R = COOCH3 1 8 R = H R = C(O)C6H5 18 R = COOH 17 Cr R ON _NOX R = H R = COOCH3 X = Br 2 3 4 5 6 7 X = Cl X = Br X = I X = CH3 X = N=C=S X = N=C=Se X = N=C=S X = N=C=Se X = Cl X = Br X = I X = CH3 9 10 11 12 13 14 R = COOH R = C(O)C6H5 15 16

2. Results and discussion

2.1. Synthesis and characterization

Bromination/nitrosylation of 8, (g5 -carboxycyclopenta-dienyl)dicarbonylnitrosylchromium (cynichrodenoic acid) (17), and (g5 -benzoylcyclopentadienyl)dicarbonylnitrosyl-chromium (benzoylcynichrodene) (18) in isopropanol, a novel method of replacing dicarbonyl with (NO)Br ligand

[9], produced 10, 15 and 16 in 84%, 74%, and 90% yield, respectively.

8HBr=ðCH3Þ2CHCH! 2CH2ONO

isopropanol 10

17! 15 18! 16

Compound 13 in 50% and 14 in 49% were obtained by treating the halogen-free solution, obtained by the action of silver nitrate on 9 in H2O/CH3OH(4/1), with excess

potassium thiocyanate and selenocyanate, respectively. 9AgNO!3 KSCN! 13 9AgNO!3 KSeCN! 14

As is well known, both thiocyanate and selenocyanate are amphoteric nucleophiles; that is, they can S-bond or N-bond to a substrate. For a soft substrate,

C N

-S S C N

-[ ] [-Se C N Se C N ]

-an S-bonded complex obtained, for a hard substrate, a N-bonded complexes ended up. It is interesting to ﬁnd which linkage, Cr–N or Cr–S(Se), is for compound 13 and 14. In 1968, Wojcicki et al. claimed the formation of 6 (CpCr(NO)2(NCS)), a N-bonded isothiocyanate complex.

They also found that with tungsten, only the S-bonded thiocyanate was isolated (CpW(CO)3(SCN)); while with

molybdenum both isomers, CpMo(CO)3(NCS) and

CpMo(CO)3(SCN) were formed. The assignments they

based were the positions of the CN stretching band, the S-bonded thiocynates(–SCN) absorbing at ca. 2120 cm1, while the N-bonded isothiocyanates(–NCS) at about 2100 cm1. The difference is small (20 cm1) and there are notable exceptions [10]. Herein, we reported the first X-ray confirmed Cr-isothiocyanato linkage com-pound, 13. The electron deficient substrate, CpCr(NO)2

+

, entails the thiocyanate to attack itself with the more basic site(nitrogen), so N-bonded isothiocyanato product was obtained. The same auguments hold for compound 14, since selenium nucleophiles are even more polarizable than sulfur analogues. On the basis of 13C NMR, 14 was assigned to a nitrogen-bonded structure, [g5-(C5H4

-COOCH3)]Cr(NO)2(N@C@Se). The isoselenocyanato

car-bon of 14 resonates at d 138.89 ppm, which is correlated well with d 138.8 from a series of organic compounds bearing a –N@C@Se substituent [11]. In the alternative linkage, selenocyanato carbon, may give a much lower va-lue of d, a vava-lue of ca. 102 ppm for organic compounds

[12].

All compounds 9–14 exhibit two terminal nitrosyl stretching bands, the symmetric mode occurring at 1799– 1844 cm1 and the asymmetric mode at 1723–1734 cm1. The absorption of organic carbonyl group is obscured by the asymmetric NO stretching band. The following order of increasing wavenumbers of symmetric NO stretching was observed: 12 (1780 cm1) < 11 (1821 cm1) < 10 (1826 cm1) < 9 (1829 cm1) < 13 (1839 cm1). This trend

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is correlated well with the order of increasing tendency of electron-withdrawing property of a coordinated ligand on the chromium atom: –CH3< I < Br < Cl < –NCS. An

elec-tron-withdrawing lignad reduces the p back-bonding from Cr dp-orbitals to the p*_{orbitals of NO}

"Cr" N O: "Cr" N O:

- +

[ ]

groups, and higher wavenumbers result. This trend rein-forces the structure assignments for 13 and 14. In 1968, Wojcicki et al. [10] revealed that for a number of C5H5M(CO)xX complexes the carbonyl stretching

frequen-cies increased with the electron-withdrawing power of X in the order: –I < Br SCN < Cl < –NCS. The above crite-rion permits unambiguous diﬀerentiation between Cr–S– C„N and Cr–N@C@S linkages. Other functional groups of these compounds show their characteristic absorbances. The1H NMR spectra of compounds 10, 13–16 are con-sistent with their assigned structures and are similar to other metallocenyl systems [3b–5] . All compounds 8–16

exhibit a pair of apparent triplet. The lowﬁeld triplet can be assigned to H(2,5) protons of the Cp. This assignment is made on the basis that the carbonyl group would exert stronger magnetic anisotropic eﬀect to the ring protons clo-ser to it. As expected, H(2,5) would be deshielded to a greater extent than the protons on the more remote 3-and 4-positions.

It is of interest to compare the1H NMR spectra of 8–13 with their corresponding unsubstituted parent compounds 1–6 (Since the1H-spectroscopic data of 7 had never been reported in the literature, the comparison of it with 14 was excluded.). The chemical shifts of protons on Cp(Cr) (H(3,4) and H(2,5)) of 8–13 occur at much lower fields than those of the corresponding protons of 1–6 (Table 1). This reflects the strong electron-withdrawing effect of the organic carbonyl group.

Table 1 1_{H NMR data}a Compound Cp(Cr) –OCH3 Cr-CH3 H(2,5) H(3,4) 1 5.07 (s, 5) 2 5.73 (s, 5) 3 5.72 (s, 5) 4 5.78 (s, 5) 5 5.40 (s, 5) 0.57 6 5.83 (s, 5) 8 5.76 (t, 2) 5.11 (t, 2) 3.80 (s, 3) 9 6.25 (t, 2) 5.78 (t, 2) 3.89 (s, 3) 10 6.26 (t, 2) 5.79 (t, 2) 3.87 (s, 3) 11 6.29 (t, 2) 5.82 (t, 2) 3.85 (s, 3) 12 5.93 (t, 2) 5.44 (t, 2) 3.79 (s, 3) 0.63 13 6.26 (t, 2) 5.77 (t, 2) 3.92 (s, 3) a

Data of 9, 11, 12 are obtained from Ref.[5].

Table 2

13_C{1_{H} NMR data}a

Compound Cp(Cr) Cr–C„O C@O –OCH3 Cr–CH3

C(1) C(2,5) C(3,4) 1 90.31 (C(1–5)) 237.10 2 103.02 (C(1–5)) 4 101.32 (C(1–5)) 5 99.24 (C(1–5)) 1.22 8 92.94 94.12 91.74 234.67 165.07 52.16 9 103.14 106.05 104.14 161.87 52.79 10 102.56 105.56 103.65 161.99 52.82 11 101.96 104.30 102.30 162.10 52.72 12 101.78 102.83 99.48 163.65 52.08 0.85 13 104.25 105.33 103.12 161.41 53.19 14 102.64 102.40 100.91 161.88 52.96 15 107.98 107.88 104.42 16 107.49 106.64 103.35 188.41 a

Chemical shifts are reported in ppm with respect to internal Me4Si.

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The assignments of 13C NMR spectra of 10, 13–16 (Table 2) were based on standard 13C NMR correlation

[13], 2D HetCOR (Figs. 1–3), the DEPT technique and by comparison with other metallo-aromatic systems [14]. In the case of 10, two relatively less intense signals were observed at d 161.99 and 102.56 corresponding to the organic carbonyl carbon and C(1) of Cp(Cr), respectively. The methoxy carbon resonates at d 52.82. The line assign-ments for C(2-5) of Cp(Cr) were more diﬃcult to make. Based on 2D-HetCOR, chemical shifts at d 103.65 and 105.56 were assigned to C(3,4) and C(2,5) of Cp(Cr), respectively (Fig. 1). Similarly, in the case of 13, chemical

shifts of d 103.12 and 105.33 were assigned to C(3,4) and C(2,5) of Cp(Cr) (Fig. 2); and in the case of 14, chemical shifts of d 100.91 and 102.40 were assigned to C(3,4) and C(2,5) of Cp(Cr), respectively (Fig. 3). It is of interest to compare the13C NMR spectra of 8–14 with their unsubsti-tuted parent compound 1–7. However, the lack data of 3, 6, and 7 from literature only the comparison of 8, 9, 11 and 12 with 1, 2, 4 and 5 were made. For the carbon atoms on Cp(Cr (C(3,4)) and C(2,5)), the chemical shifts occur at lower field than the chemical shifts of their parent com-pounds (Table 3). This reflects the strong electron-with-drawing effect of carbonyl substituent on the Cp ring.

One surprising finding in the study of13C spectra of 8– 16 (Table 2) is that the highfield and lowfield chemical shifts are assigned to C(3,4) and C(2,5), respectively for electron-withdrawing carbonyl substituent on Cp ring which is opposite to the assignment of ferrocene derivatives

[14,9] in which the lowﬁeld shifts and highﬁeld shifts are assigned to C(3,4) and C(2,5). In ferrocenes, the 3,4-posi-tions of the substituted cyclopentadienyl ring are more sen-sitive to electron-withdrawing substituents by resonance, while in cynichrodenes the 2,5-positions of the substituted cyclopentadienyl ring are more sensitive to electron-with-drawing substituents. Ii + Cr ON _NOX C _O -IIi + C _O -Fe Cr ON _NOX C _O I II Fe O C

The smaller contribution of canonical form Ii than IIii to each of the corresponding structures I and II may explain such behaviour. This is understandable in the destabilization of chromium cation because of the overall electron-withdrawing properties of two NO and X ligands. Therefore, in cyclopentadienyl chromium complexes, CpCr(NO)2X, bearing a electron-withdrawing substituent,

the inductive eﬀect that deshields the nearby carbon (C(2,5)) atoms to a greater extent than the more distant 3- and 4-positions may explain the observed data collected in Table 2(C13 Table).

An important advantage of the13C NMR method over

1

H NMR spectroscopy is the relatively lower susceptibility of 13C chemical shifts to the eﬀects of magnetically

aniso-Table 3a

Selected net atomic charges for 10 using the 6-311++G(d,p) basis set C(1) 0.18290 C(2) 0.11927 C(3) 0.22087 C(4) 0.18500 C(5) 0.17839 Table 3b

Selected net atomic charges for 13 using the 6-311++G(d,p) basis set C(1) 0.19908 C(2) 0.15829 C(3) 0.18701 C(4) 0.19716 C(5) 0.11584

Fig. 2. 2D1_H{13_{C} HetCOR NMR spectrum of 13 in CDCl} 3.

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tropic groups and ring current[15]. Therefore, 13C NMR spectra provide a clearer picture of the electron density dis-tribution within a molecule than do proton NMR spectra. Thus, to obtain the unequivocal assignments of C(2,5) and C(3,4) on the Cp ring, the use of 2D HetCOR NMR spec-troscopy is highly recommended, especially for metals coordinated with ligands bearing strong electron-with-drawing property.

The unequivocal assignments of13C chemical shifts for 10 and 13 were correlated well with the ab initio calcula-tions from the X-ray data of 10 and 13. The average charges of C(2,5) and C(3,4) are 0.149 and 0.203 for 10; and0.137 and 0.192 for 13 (Tables 3a and 3b).

The molecular structure of 10 and 13 are shown inFigs. 4 and 5, respectively. Selected bond distances and angles are given in Tables 4 and 5. The atomic coordinates of the non-hydrogen atoms are listed in Tables 6 and 7, respectively.

The coordination geometry about the Cr center in each case is approximately a distorted tetrahedron with two nitrosyl groups, the Cp group and X (X = bromide for 10, and isothiocyanate for 13) as the four coordination sites. It is worth pointing out that for all the structures of 9[5], 10, 11[5], and 13, one of the nitrosyl groups is located at the site away from the exocyclic carbon atom of Cr(Cr). (Namely, the X group is located at the site toward the

exo-Fig. 4. Molecular conﬁguration of 10.

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cyclic carbon). In compound 10, the twist angle is 168.5 (Fig. 6), and in 13, the twist angle is 172.3 (Fig. 7). The twist angle is deﬁned as the torsional angle between the nitrosyl nitrogen atom(N(2)), the chromium atom, the Cp center and the ring carbon atom bearing the exocyclic car-bon atom. The preference for the unsymmetrical isomer i to the symmetrical isomer ii may be related to the ability of the exocyclic double bond to donate electron density to the chromium atom such that it is trans to the better p-accepting ligand, i.e. NO+.

Ii + Cr ON _NOX C _O -Iii + Cr X _NONO C _O -Cr X NO NO i Cr ON NO X ii

As a result, the exocyclic carbons C(6) of 10 and 13 are bent towards the chromium atom with h angles of 0.4 and 0.6, respectively. The h angle is deﬁned as the angle between the exocyclic C–C bond (C(1)–C(6)) and the corre-sponding Cp ring with a positive angle toward metal and a negative angle away from the metal. The result is startling,

Table 4

Selected bond length (A˚ ) and selected bond angles () for 10

Cr–C(1) 2.205(3) Cr–C(2) 2.249(3) Cr–C(3) 2.229(4) Cr–C(4) 2.198(3) Cr–C(5) 2.192(3) C(1)–C(2) 1.410(5) C(1)–C(5) 1.418(5) C(2)–C(3) 1.393(5) C(3)–C(4) 1.413(5) C(4)–C(5) 1.403(5) Cr–N(1) 1.714(3) Cr–N(2) 1.717(3) Cr–Br 2.4563(7) N(1)–O(1) 1.153(4) N(2)–O(2) 1.154(4) C(1)–C(6) 1.472(5) C(6)–O(3) 1.199(4) C(6)–O(4) 1.324(4) C(7)–O(4) 1.438(5) Cr. . .C(6) 3.248 Cr–Cp 1.863 O(1). . .Br 4.083 O(2). . .Br 4.094 N(1)–Cr–N(2) 94.76(14) N(1)–Cr–Br 97.61(10) N(2)–Cr–Br 97.47(11) O(1)–N(1)–Cr 173.5(3) O(2)–N(2)–Cr 172.9(3) C(2)–C(1)–C(6) 125.0(3) C(5)–C(1)–C(6) 126.9(3) O(3)–C(6)–O(4) 123.8(3) O(3)–C(6)–C(1) 124.6(3) O(4)–C(6)–C(1) 111.6(3) C(6)–O(4)–C(7) 117.9(3) Table 5

Selected bond length (A˚ ) and selected bond angles () for 13

Cr–C(1) 2.216(3) Cr–C(2) 2.189(3) Cr–C(3) 2.190(3) Cr–C(4) 2.225(3) Cr–C(5) 2.251(3) C(1)–C(2) 1.427(4) C(1)–C(5) 1.410(5) C(2)–C(3) 1.398(5) C(3)–C(4) 1.412(5) C(4)–C(5) 1.397(4) Cr–N(2) 1.717(2) Cr–N(1) 1.717(3) Cr–N(3) 1.983(3) S(1)–C(8) 1.622(3) N(1)–O(1) 1.158(3) N(2)–O(2) 1.158(3) N(3)–C(8) 1.154(4) O(3)–C(6) 1.194(4) O(4)–C(6) 1.334(4) O(4)–C(7) 1.444(4) C(1)–C(6) 1.486(4) Cr. . ..C(6) 3.283 Cr–Cp 1.861 O(1). . .S 5.970 O(2). . .S 5.860 N(1)–Cr–N(2) 93.69(13) N(1)–Cr–N(3) 99.45(13) N(2)–Cr–N(3) 98.46(12) O(1)–N(1)–Cr 170.8(3) O(2)–N(2)–Cr 171.9(2) C(8)–N(3)–Cr 171.5(3) C(6)–O(4)–C(7) 116.1(3) C(5)–C(1)–C(6) 124.3(3) C(2)–C(1)–C(6) 128.2(3) O(3)–C(6)–O(4) 125.2(3) O(3)–C(6)–C(1) 123.5(3) O(4)–C(6)–C(1) 111.3(3) N(3)–C(8)–S(1) 178.2(3) Table 6 Atomic coordinates (·104

) and equivalent isotropic displacement param-eters (A˚2· 103 ) for 10 x y z U(eq) Cr 3849(1) 1522(1) 2448(1) 33(1) Br 746(1) 2104(1) 2768(1) 56(1) N(1) 3616(4) 1354(2) 381(4) 41(1) N(2) 5169(4) 2293(2) 2236(4) 44(1) C(1) 3381(5) 413(2) 3320(4) 37(1) C(2) 3087(6) 874(2) 4662(4) 45(1) C(3) 4779(6) 1243(2) 5018(4) 48(1) C(4) 6153(5) 1004(2) 3921(5) 46(1) C(5) 5300(5) 488(2) 2871(4) 40(1) C(6) 1932(5) 70(2) 2565(4) 41(1) C(7) 1401(7) 976(2) 550(6) 70(1) O(1) 3585(4) 1190(2) 984(3) 60(1) O(2) 6219(5) 2766(2) 2102(4) 71(1) O(3) 331(4) 125(2) 3006(4) 80(1) O(4) 2620(4) 447(1) 1341(4) 56(1)

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since it overthrows the concept that cyclopentadienyl groups with the powerful electron-withdrawing groups have them oriented trans to the NO ligand[19].

It is interesting to ﬁnd out that the contribution of canonical form Ii to I to some extent was revealed by the carbon–carbon bond lengths in the cyclopentadienyl ring. The shorter bond lengths in A˚ of C(2)–C(3) (1.393(5), 1.398(5)) and C(4)–C(5) (1.403(5), 1.397(4)), and longer bond lengths of C(1)–C(2) (1.410(5), 1.427(4)), C(3)–C(4) (1.413(5), 1.412(5)) and C(1)–C(5) (1.418(5), 1.410(5)) were obtained in both cases of 10 and 13.

In 13, the chromium-isothiocyanate fragment is approx-imately linear, with Cr–N–C and N–C–S angles of 171.5(3) and 178.2(3), respectively. Given that the metal-N–C angle is linear, a predominant weight was

assigned to canonical form iii and a relatively insigniﬁcant weight to iv[20]. iii M N C S + -[ N C M S ] iv

In view of the shortness of the Cr–N(nitrosyl) distances (in A˚ ) (Cr–N(1), 1.717(3); Cr–N(2), 1.717(2)) relative to Cr–N(isothiocyanato) (Cr–N(3), 1.983(3)) for compound 13, appreciable dp back-donation from chromium atom into the p* _{orbitals of nitrosyl group is demonstrated.}

And it appears that the canonical form vi rather than v has a greater extent of contribution to the chromium-nitrosyl v [ ] vi vii + -2 M N O + M N- + O N O M

bonding. And the less contribution of vii than vi was re-ﬂected by the angles of Cr–N–O of ca. 171 (Cr–N(1)– O(1), 170.8(3), Cr–N(2)–O(2), 171.9(2)). These values are similar to those found in 9 (171.2(5), 172.1(4)) and 11 (176.0(5) and 174.3(4)), respectively[5].

3. Experimental details

All the syntheses were carried out under nitrogen by the use of Schlenk techniques. Traces of oxygen in the nitrogen were removed with BASF catalyst and deoxygenated nitro-gen was dried over molecular sieves (3 A˚ ) and P2O5.

Hex-ane, pentHex-ane, benzene, and dichloromethane were dried over calcium hydride and freshly distilled under nitrogen. Diethyl ether was dried over sodium and redistilled under

Table 7

Atomic coordinates (·104

) and equivalent isotropic displacement param-eters (A˚2· 103 ) for 13 x y z U(eq) Cr 4225(1) 1179(1) 2116(1) 33(1) S(1) 9031(2) 3363(1) 2304(1) 62(1) N(1) 3093(4) 743(3) 3870(3) 42(1) N(2) 2467(4) 462(3) 1561(3) 38(1) N(3) 6426(4) 599(3) 2132(3) 45(1) O(1) 2169(4) 639(3) 5010(3) 69(1) O(2) 1155(4) 154(3) 1166(3) 59(1) O(3) 8969(3) 2617(3) 2974(3) 54(1) O(4) 6041(3) 3261(3) 4375(3) 51(1) C(1) 5875(4) 3045(3) 2116(3) 37(1) C(2) 3757(5) 3602(4) 2177(4) 42(1) C(3) 3223(5) 3503(4) 924(4) 48(1) C(4) 4977(5) 2895(4) 81(4) 47(1) C(5) 6604(5) 2631(4) 807(4) 41(1) C(6) 7162(5) 2936(4) 3183(4) 40(1) C(7) 7149(6) 3304(5) 5449(4) 62(1) C(8) 7519(5) 1744(4) 2220(3) 39(1)

U(eq)is deﬁned as one-third of the trace of the orthogonalized Uijtensor.

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nitrogen from sodium-benzophenone ketyl. All the other solvents were used as commercially obtained.

Column chromatography was carried out under nitro-gen with Merck Kiesel-gel 60. The silica gel was heated with a heat gun during mixing in a rotary evaporator attached to a vacuum pump for 1 h to remove water and oxygen. The silica gel was then stored under nitrogen until use. Compounds 17[9]and 18, were prepared according to the literature procedures[16].

1

H and13C NMR were acquired on a Varian Unity-300 spectrometer. Chemical shifts were referenced to tetra-methylsilane. IR spectra were recorded a Perkin-Elmer Fourier transform IR 1725X spectrophotomer. Microanal-yses were carried out by the Microanalytic Laboratory of the National Chung Hsing University.

3.1. Preparation of bromo(g5-carbomethoxycyclopentadienyl) dinitrosylchromium (10)

Through a solution of (g5 -Carbomethoxycyclopentadie-nyl)dicarbonylnitrosylchromium (8) (2.25 g, 8.65 mmol) in 20 ml of isopropanol, hydrogen bromide was bubbled for 5 min. After cooling to 0–10C with stirring for 15 min (orange red solution resulted), isoamyl nitrite (2.2 ml, 16.36 mmol) was added slowly. Carbon oxide evolved and the solution changed to dark green abruptly. The reaction mixture was continued to stir for 1 h. The solvent was removed. The residue was extracted with 100 ml of dichlo-romethane. The extract was washed several times with dis-tilled water until the aqueous layer gave an light yellow color and dried with magnesium sulfate. The solution was

ﬁltered and concentrated to dark-green solid. The solid was dissolved in 25 ml of dichloromethane. Five grams of silica gel were added and the solvent was then removed under vacuum. The residue was added to a dry-packed col-umn (2.0· 10 cm) of silica gel. Elution of the column with hexane/ether (2:1) gave an brownish green layer which upon removal of the solvent gave bromo(g5 -carbomethoxycyclo-pentadienyl)dinitrosylchromium (10) (2.29 g, 84%). An X-ray sample (elongated gleamy blackish brown crystal) was prepared by recrystallization using the solvent expansion method from hexane:dichloromethane (8:3) at 0C for 48 h.

Anal. Calc. for C7H7O4N2BrCr: C, 26.68; H, 2.24; N,

8.89. Found: C, 26.80; H, 2.37; N, 8.44%. Proton NMR (CDCl3): d (relative intensity, multiplicity, assignment):

3.87 (3H, s, –OCH3); 5.79 (2H, t, Cp H(3,4)); 6.26 (2H, t, Cp H(2,5)). Carbon-13 NMR (CDCl3): d (assignment): 52.82 (–OCH3); 102.56(Cp, C(1)); 103.65 (Cp, C(3,4)); 105.56 (Cp, C(2,5)); 161.99 (–COOCH3). IR (KBr): m (cm1) (intensity): 3082 (s), 1826(vs), 1731(bvs), 1475 (s), 1424 (s), 1293(s), 1149(s), 858(s), 776 (s). Mass spectrum: m/z 315(M+). 3.2. Preparation of (g5-carbomethoxycyclopentadienyl) (isothiocyanato)dinitrosylchromium (13) To a solution of (g5 -carbomethoxycyclopentadie-nyl)chlorodinitrosylchromium (9) (0.13 g, 0.48 mmol) in 20 ml H2O 5 ml CH3OH, silver nitrate (0.09 g, 0.60 mmol)

was added. The solution was stirred for 30 min at room temperature, followed by ﬁltration to remove silver

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ride. To the ﬁltrate, potassium thiocyanate (0.09 g, 0.96 mmol) was added and the reaction mixture was stirred for 8 h at room temperature. The reaction mixture was then extracted with dichloromethane. The extract was washed three times with distilled water and dried with anhydrous magnesium sulfate. The solution was ﬁltered. Twenty grams of silica gel were added to the solution, and the solvent was then removed under vacuum. The res-idue was added to a dry-packed column (2.0· 10 cm) of silical gel. Elution of the column with hexane:ether (3:1) gave a dark green band which upon removal of the solvent gave (g5 -carbomethoxycyclopentadienyl)(isothiocyanato)-dinitrosylchromium (13) (0.07 g, (50%)) as a dark green solid. An X-ray sample (granular blackish brown crystal) was prepared by recrystallization using the solvent expan-sion method from hexane:chloroform (8:3) at 0C for 48 h. Anal. Calc. for C8H7O4N3SCr: C, 32.77; H, 2.41; N,

14.33. Found: C, 33.28; H, 2.66 N, 14.08%. Proton NMR (CDCl3): d (relative intensity, multiplicity, assignment):

3.92 (–OCH3); 5.77 (2H, t, Cp H(3,4)); 6.26 (2H, t, Cp H(2,5)). Carbon-13 NMR (CDCl3): d (assignment): 53.19 (–OCH3); 92.39 (Cr-NCS); 103.12 (Cp, C(3,4)); 104.25 (Cp, C(1)); 105.33 (Cp, C(2,5)); 161.41 (–COOCH3). IR(KBr): m (cm1) (intensity): 2112(s), 1839(s), 1723(bvs), 1425(m), 1297(s), 1153(s), 845(m), 584(m). Mass spectrum: m/z 293 (M+). 3.3. Preparation of (g5-carbomethoxycyclopentadienyl) (isoselenocyanato)dinitrosylchromium (14) To a solution of (g5 -carbomethoxycyclopentadie-nyl)chlorodinitrosylchromium (8) (0.13 g, 0.48 mmol) in 20 ml H2O 5 ml CH3OH, silver nitrate (0.1 g, 0.60 mmol)

was added. The solution was stirred for 30 min at room temperature, followed by filtration to remove silver chlo-ride. To the filtrate, potassium selenocyanate (0.14 g, 0.96 mmol) was added and the reaction mixture was stir-red for 12 h at room temperature. The reaction mixture was then extracted with dichloromethane. The extract was washed three times with distilled water and dried with anhydrous magnesium sulfate. The solution was fil-tered. Twenty grams of silica gel were added to the solu-tion, and the solvent was then removed under vacuum. The residue was added to a dry-packed column (2.0· 10 cm) of silical gel. Elution of the column with dichloromethane gave a green band which upon removal of the solvent gave (g5 -carbomethoxycyclopentadie-nyl)(isoselenocyanato)dinitrosylchromium (14) (0.08 g, (49%)) as a green solid.

Anal. Calc. for C8H7O4N3SeCr: C, 28.25; H, 2.07; N,

12.36. Found: C, 28.75; H, 2.40 N, 12.65%. Proton NMR (CDCl3): d (relative intensity, multiplicity, assignment):

3.88 (–OCH3); 5.75 (2H, t, Cp H(3,4)); 6.25 (2H, t, Cp H(2,5)). Carbon-13 NMR (CDCl3): d (assignment): 52.96 (–OCH3); 100.91 (Cp, C(3,4)); 102.40 (Cp, C(2,5)); 102.64 (Cp, C(1)); 138.89 (Cr-NCSe); 161.88 (–COOCH3). IR (KBr): m (cm1) (intensity): 3398(bs), 2163(w),1844(s), 1734(bvs), 1302(s), 1097(s), 620(s). Mass spectrum: m/z 340 (M+).

3.4. Preparation of bromo(g5-carboxycyclopentadienyl) dinitrosylchromium (15)

Through a solution of (g5 -carboxycyclopentadie-nyl)dicarbonylnitrosylchromium (17) (2.49 g, 9.56 mmol) in 30 ml of isopropanol, hydrogen bromide was bubbled for 5 min. After cooling to 0–10C with stirring for 20 min (orange red solution resulted), isoamyl nitrite (2.6 ml, 19.12 mmol) was added slowly. Carbon oxide evolved and the solution changed to dark green subse-quently. The reaction mixture was continued to stir for 1 h. After concentration of the solution to 10 ml, 30 ml of dichloromethane was added, a large quantity of dark green solid precipitated out. The solid was obtained through frit ﬁltration and were washed several times with distilled water. Compound bromo(g5 -carboxycyclopenta-dienyl)dinitrosylchromium (15) (2.24 g, 74%) was obtained after vacuum drying. An X-ray sample (granular blackish brown crystal) was prepared by recrystallization using the solvent expansion method from hexane:tetrahy-drofuran (5:2) at 0C for 48 h.

9.31. Found: C, 24.04; H, 1.73; N, 8.77%. Proton NMR (CD3COCD3): d (relative intensity, multiplicity,

assign-ment): 6.02 (2H, t, Cp H(3,4)); 6.41 (2H, t, Cp H(2,5)). Car-bon-13 NMR (CD3COCD3): d (assignment): 107.98 (Cp,

C(1)); 104.42 (Cp, C(3,4)); 107.88 (Cp, C(2,5)). IR (KBr): m (cm1) (intensity): 3109–2368 (m, broad), 1832(vs), 1730(vs), 1683 (vs), 1486 (s), 1304 (s), 1178(s), 858 (s), 587 (s). Mass spectrum: m/z 315 (M+). 3.5. Preparation of (g5-benzoylcyclopentadienyl) bromodinitrosylchromium (16)

Through a solution of (g5 -benzoylcyclopentadienyl)dic-arbonylnitrosylchromium (18) (1.23 g, 4 mmol) in 50 ml of isopropanol, hydrogen bromide was bubbled for 5 min. After cooling to 0–10C with stirring for 15 min (orange red solution resulted), isoamyl nitrite (2.0 ml, 14.48 mmol) was added slowly. Carbon oxide evolved and the solution changed to dark green subsequently. The reaction mixture was continued to stir for 1 h. The solvent was removed. The residue was extracted with 100 ml of dichloromethane. The extract was washed several times with distilled water until the aqueous layer gave a light yellow color and dried with magnesium sulfate. The solution was ﬁltered and con-centrated to dark-green solid. The solid was dissolved in 25 ml of dichloromethane. Five grams of silica gel were added and the solvent was then removed under vacuum. The residue was added to a dry-packed column (1.8· 9 cm) of silica gel. Elution of the column with hex-ane/ether (3:1) gave an brownish green layer which upon removal of the solvent gave (g5 -benzoylcyclopentadie-nyl)bromodinitrosylchromium (16) (1.24 g, 90%). An

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X-ray sample (granular gleamy blackish brown crystal) was prepared by recrystallization using the solvent expansion method from hexane:dichloromethane (5:2) at 0C for 48 h.

7.76%. Proton NMR (CDCl3): d (relative intensity,

multi-plicity, assignment): 5.88 (2H, t, Cp H(3,4)); 6.32 (2H, t, Cp H(2,5)); 7.51 (2H, t, Ph H(3,5)); 7.60 (1H, t, Ph H(4)); 7.85 (2H, d, Ph H(2,6)). Carbon-13 NMR (CDCl3): d (assignment): 103.35 (Cp, C(3,4)); 106.64(Cp, C(2,5)); 107.49 (Cp, C(1)); 128.75(Ph, C(3,5)); 128.83 (Ph, C(2,6)); 133.36 (Ph, C(4)); 136.91 (Ph, C(1)); 188.41 (–C(O)–). IR (KBr): m (cm1) (intensity): 1828(vs), 1739(vs), 1643(s), 1287(s), 853(s). Mass spectrum: m/z 361 (M+).

3.6. X-ray diﬀraction analyses of 10 and 13

The intensity data were collected on a CAD-4 diffrac-tometer with a graphite monochromator (Mo Ka radia-tion) for compound 10; on SMART-CCD diffractometer using x scan for compound 13. h–2h scan data were col-lected at room temperature (24C). The data were cor-rected for absorption, Lorentz and polarization effects. The absorption correction is according to the empirical psi rotation. The details of crystal data and intensity

collec-tion are summarized inTables 8 and 9for compounds 10 and 13, respectively.

The structures were solved by direct methods and were refined by full matrix least squares refinement based on F values. All of the non-hydrogen atoms were refined with anisotropic thermal parameters. All of the hydrogen atoms were positioned at calculated coordinate with a fixed isotro-pic thermal parameter (U = U(attached atom) + 0.01 A˚2). Atomic scattering factors and corrections for anomalous dispersion were from International Tables for X-ray Crystal-lography [17]. All calculations were performed on a PC computer usingSHELEXsoftware package[18].

3.7. Computational method

Here, we use unrestricted B3LYP hybrid method involv-ing the three-parameter Becke exchange functional[21]and a Lee–Yang–Parr correlation functional [22]. All calcula-tions are performed using GAUSSIAN-03 program [23]. The

geometries for 10 and 13 are taken from the crystallo-graphic data. The atomic charges have been analyzed using the natural population analysis (NPA) which yields reliable atomic charges and natural bond orbital (NBO)

calcula-Table 8

Selected crystal data and reﬁnement parameters for 10 Empirical formula C7H7BrN2O4Cr

Formula weight (g/mol) 315.06

Temperature (K) 293(2)

Wavelength (A˚ ) 0.71073

Crystal system Monoclinic

Space group P2(1)/n

Unit cell dimensions

a (A˚ ) 7.0276(8) b (A˚ ) 18.5427(14) c (A˚ ) 8.1481(7) b() 92.231(8) Volume (A˚3) 1060.98(17) Z 4 Dcalc(Mg/m 3 ) 1.972 Absorption coeﬃcient (mm1) 4.839 F(0 0 0) 616 Crystal size (mm3) 0.50· 0.25 · 0.22 hRange for data collection () 2.20–27.50

Index ranges 9 6 h 6 9, 0 6 k 6 24,

0 6 l 6 10

Reﬂections collected 2439

Independent reﬂections [Rint] 2439 [0.0000] Completeness to h = 27.50 100.0%

Absorption correction W-scan

Maximum and minimum transmission 0.3155 and 0.2727 Reﬁnement method full-matrix

least-squares on F2

Data/restraints/parameters 2439/0/137

Goodness-of-ﬁt on F2 1.010

Final R indices [I > 2r(I)] R1= 0.0337, wR2= 0.0876 R indices (all data) R1= 0.0664, wR2= 0.0966

Extinction coeﬃcient 0.0026(8)

Largest diﬀerence in peak and hole (e A˚3)

0.418 and0.624

Table 9

Selected crystal data and reﬁnement parameters for 13 Empirical formula C8H7N3O4SCr

Formula weight (g/mol) 293.23

Temperature (K) 295(2)

Wavelength (A˚ ) 0.71073

Crystal system Triclinic

Space group P 1

Unit cell dimensions

a (A˚ ) 6.8128(4) b (A˚ ) 9.1295(6) c (A˚ ) 9.9184(7) a() 74.3400(10) b() 77.5200(10) c() 78.787(2) Volume (A˚3) 573.85(6) Z 2 Dcalc(Mg/m 3 ) 1.697 Absorption coeﬃcient (mm1) 1.184 F(0 0 0) 296 Crystal size (mm3) 0.20· 0.12 · 0.04 hRange for data collection () 2.17–26.37

Index ranges 8 6 h 6 8, 11 6 k 6 11,

12 6 l 6 12

Reﬂections collected 5433

Independent reﬂections [Rint] 2273 [0.0353] Completeness to h = 26.37 96.5%

Absorption correction Semi-empirical from equivalents Maximum and minimum transmission 0.8944 and 0.7368 Reﬁnement method full-matrix

least-squares on F2

Data/restraints/parameters 2273/0/155 Goodness-of-ﬁt on F2 1.034

Final R indices [I > 2r(I)] R1= 0.0436, wR2= 0.0898 R indices (all data) R1= 0.0646, wR2= 0.0973 Largest diﬀerence in peak and hole

(e A˚3)

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tions[24]. An important feature of the NBO method is that the presence of diﬀuse functions in the basis sets does not aﬀect the results.

Acknowledgements

The authors are grateful to the National Science Council of Taiwan for grants in supports of this research program and the computational resources provided National Center for High-Performance Computing.

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