Zonal management of multi-purpose use of water from arsenic-affected aquifers by using a multi-variable indicator kriging approach

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Syntheses and characterizations of group 6 metal

cyanotrihydroborate complexes

q

Fu-Chen Liu

a,*

, Yow-Chuan Sheu

a

, Jo-Ju She

a

, Yu-Chang Chang

b

,

Fung-E Hong

b

, Gene-Hsian Lee

c

, Shie-Ming Peng

c

a_{Department of Chemistry, National Dong Hwa University, Hualien 974, Taiwan, ROC} b_{Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan, ROC}

c_{Department of Chemistry, National Taiwan University, Taipai 106, Taiwan, ROC}

Received 21 August 2003; accepted 13 November 2003

Abstract

The anions, [M(CO)6–n(NCBH3)n]n(n¼ 2, M ¼ Cr(1); n ¼ 3, M ¼ Cr(2), Mo(3), W(4)), were prepared either from the reactions

of sodium cyanotrihydroborate with group 6 transition metal hexacarbonyls, M(CO)6(M¼ Cr, Mo, W), or through the reactions of

M(CO)3(CH3CN)3(M¼ Cr, W) with sodium cyanotrihydroborate. The cyanotrihydroborate ligand bonds to the metal through a

nitrogen atom, which was confirmed by the Infrared, proton and boron NMR spectroscopies. Crystal structures of the above complexes were determined by single crystal X-ray diffraction analyses. A cis configuration is found in 1. Molecular structures of 2, 3, and 4 are similar and a facial configuration is observed.

Keywords: Cyanotrihydroborate; Organohydroborate; Hydroborate; Crystal structure; Hexacarbonyl

1. Introduction

Sodium cyanotrihydroborate is a mild reducing agent; it plays an important role in organic and inor-ganic syntheses [1]. It also displays versatile chemistry in the formation of transition metal complexes. Many cy-anotrihydroborate metal complexes have been prepared and the bonding interactions between the metal and the cyanotrihydroborate ligand have been studied [2–4]. The cyanotrihydroborate anion uses either the nitrogen atom, or both the nitrogen and the BH hydrogen atoms to bond to the metal, thus, two kinds of bonding

in-teractions; M–NCBH3and M–NCBH2H–M, have been

observed. In addition, cleavage of the NC–BH3bond to

form a CN containing compounds [3c,3d,4e] or

forma-tion of the isomeric M–CNBH3 compounds [3c] have

also been reported. In most cases Infrared spectroscopy

has been the only tool employed to identify the bonding mode between the metal and the cyanotrihydroborate anion [3]. To our knowledge, few cyanotrihydroborate complexes [3h,3i,4] have been structurally characterized through the X-ray diﬀraction technique. Although much has been published concerning cyanotrihydroborate chemistry, these previous studies were focused on late transition metal complexes, and only few examples of early transition metals have been studied [4g,5]. In the present work we report the preparations, properties, and the structural characterizations of group 6 metal cya-notrihydroborate complexes.

2. Results and discussion

2.1. Preparations of [M(CO)6–n(NCBH3)n]n (n¼ 2,

M¼ Cr(1); n ¼ 3, M ¼ Mo(3), W(4))

The complexes [N(CH3)4]n[M(CO)6–n(NCBH3)n]

(n¼ 2, M ¼ Cr(1); n ¼ 3, M ¼ Mo(3), W(4)) were

pre-pared from the reactions of M(CO)6 (M¼ Cr, Mo, W)

www.elsevier.com/locate/jorganchem

q

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jorganchem.2003.11.005.

*

Corresponding author. Tel.: +886-3-8633601; fax: +886-3-8633570. E-mail address:[email protected](F.-C. Liu).

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with two or three equivalents of NaBH3CN, followed by

the metathesis of the [Na]þ cation by the [N(CH3)4]þ

cation in the 1,2-dimethoxyethane (DME) solvent, as

shown in Eqs. (1) and (2). In the reaction of Mo(CO)6

with three equivalents of NaBH3CN, complex 3 was

the only isolated product. However, the

cyanotrihyd-roborate di- and tri-substituted compounds [Na]n[M

(CO)6–n(NCBH3)n] (n¼ 2, 3; M ¼ Cr, W) were obtained

when the sodium cyanotrihydroborate reacted with chromium carbonyl or tungsten carbonyl, and the major product was the cyanotrihydroborate disubstituted

complexes [Na]2[M(CO)4(NCBH3)2], (M¼ Cr, W). The

cyanotrihydroborate disubstituted complex 1 was iso-lated through the extraction of the reaction mixture by DME solvent. The cyanotrihydroborate trisubstituted complex 4 was obtained through fractional

crystalliza-tion from the DME/CH3CN mixed solvent. A low yield

of complex 4 was obtained.

MðCOÞ6þ nNa½BH3CN

!

DME

½Na_n½MðCOÞ_6–nðNCBH3Þn þ nCO

ðn ¼ 2; M ¼ Cr; n ¼ 3; M ¼ Mo; WÞ ð1Þ ½Na_n½MðCOÞ_6–nðNCBH3Þn þ n½ðCH3Þ4NCl ! DME ½ðCH3Þ4Nn½MðCOÞ6–nðNCBH3Þn þ nNaCl ðn ¼ 2; M ¼ Cr; n ¼ 3; M ¼ Mo; WÞ ð2Þ 2.2. Preparations of [M(CO)3(NCBH3)3]3 (M¼ Cr(2), W(4))

Complexes 2 and 4 could be obtained in good yield

from the reaction of M(CO)3(CH3CN)3(M¼ Cr, W) [6]

with three equivalents of NaBH3CN in acetonitrile,

followed by a metathesis of the [Na]þ cation by the

[N(CH3)4]þcation in DME solvent, as shown in Eqs. (3)

and (4). It is obvious that the coordinating ability of the cyanotrihydroborate anion to the metal center is far better than that of the acetonitrile, as the acetonitrile was used in these reactions.

MðCOÞ3ðNCCH3Þ3þ 3Na½BH3CN ! CH3CN ½Na3½MðCOÞ3ðNCBH3Þ3 þ 3CH3CN ðM ¼ Cr; WÞ ð3Þ ½Na₃½MðCOÞ₃ðNCBH3Þ3 þ 3½ðCH3Þ4NCL ! DME ½ðCH3Þ4N3½MðCOÞ3ðNCBH3Þ3 þ 3NaCl ðM ¼ Cr; WÞ ð4Þ

Complexes 1, 2, 3, and 4 were crystallized either from

a CH3CN/THF or CH3CN/DME mixed solvent. A

DME molecule co-crystallized with complexes 2, 3, and 4. Complex 1 is stable in air for weeks, however, com-plexes 2, 3, and 4 are very air sensitive and decompose in

the air within seconds. Complex 1 is very soluble in acetonitrile and it is slightly soluble in DME, THF, and diethyl ether, however, the cyanotrihydroborate trisub-stituted complexes 2, 3, and 4 are only soluble in acetonitrile.

2.3. NMR and infrared spectral studies

The boron resonances of complexes 1, 2, 3, and 4

appear in the range )41.5 to )42.9 ppm with a B–H

coupling constant of 89 Hz. In the proton spectra, the

resonances of the BH3 hydrogens of these complexes

appear in the range 0.30–0.37 ppm as a broad quartet. These chemical shifts and coupling constants are

consistent with those found in NaBH3CN ()43.5 ppm,

JB–H¼ 89.0 ppm) [7] and those reported in the

cya-notrihydroborate complexes [2d,4a,4b,5b], which sug-gest that the coordination of the [BH3CN]ligand to the

metal is through a nitrogen atom. Although the boron chemical shifts of the cyanohydroborate-bridged com-plexes have not been reported, signiﬁcant changes have been observed in other organohydroborate ligands upon coordinating of the B–H hydrogen to the metal [8]. The narrow ranges of the chemical shifts of the boron and

the BH3 hydrogen atoms in complexes 1, 2, 3, and 4

indicate little change of the electron density on the bo-ron and the hydrogen nuclei upon coordination of the nitrogen to the metal.

Table 1 lists the Infrared data of the complexes 1, 2,

3, 4, [N(CH3)4][NCBH3], and Na[NCBH3]. These

ab-sorptions were assigned in accord with assignments in the literature [3,4]. In earlier studies Infrared spectros-copy is the major tool to identify the bonding modes between the metal and the cyanotrihydroborate ligand. The absorption bands of the cyanotrihydroborate ligand in complexes were often compared with those observed in sodium cyanotrihydroborate. As shown in Table 1, there are signiﬁcant diﬀerences in the absorption bands

between the Na[BH3CN] and the [N(CH3)4][BH3CN].

Obviously, the counterion eﬀect of the cation plays an important role in these two salts, and we believe that comparison of these bands between complexed cya-notrihydroborates (most of them are neutral

com-pounds) and sodium cyanotrihydroborate in the

literature are not relevant.

Three terminal mBHbands are found in each complex.

As shown in Table 1, these absorption bands do not

shift signiﬁcantly from those found in complex

[N(CH3)4][NCBH3]. This result is consistent with that

found in the NMR study, where little change of the BH3

chemical shifts was found upon coordination of the ni-trogen to the metal. Two CBN absorption bands are observed in each complex, while one absorption band

(about 2230 cm1) is at about the same position as that

of the free ion, the other band has a blue shift of 17–25

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common for coordinated cyanotrihydroborate ligands [3,4] and is attributed to the increasing r bonding be-tween the carbon and the nitrogen [9], where a weakly

bound M–NCBH3 is formed.

As shown in Table 1, Infrared spectra reveal four carbonyl bands for complex 1 and two carbonyl bands for each cyanotrihydroborate trisubstituted complex 2, 3, and 4. Each cyanotrihydroborate trisubstituted complex has a low energy carbonyl band compared to complex 1. This low energy shift of the cyanotrihyd-roborate trisubstituted complex is due to the weak p back bonding ability of the cyanotrihydroborate ligand [10], where the metal transfers more electron density to

the p _{antibonding orbital of the carbonyl ligands and}

weaken the C@O bonds, resulting in a low energy shift

of the carbonyl bands. 2.4. Molecular structures

The molecular structures of 1, 2, 3, and 4 were de-termined by single-crystal X-ray diﬀraction analyses. The molecular structures of 2, 3, and 4 are similar to each other. The anions of 1 and 2 are shown in Figs. 1 and 2, and the molecular structures of 3 and 4 are in-cluded in the supporting information. The crystallo-graphic data and selected positional parameters, bond

distances, and bond angles of 1, 2, 3, and 4 are given in Tables 2–6. To our knowledge, there is only one struc-turally characterized example of early transition metal cyanotrihydroborate complex been reported [4g]. The molecular structures of these anions are best described as distorted octahedral coordination. The corners of the octahedra are either occupied by a carbon atom or a nitrogen atom. A cis conﬁguration of the cyanohyd-roborate ligands was observed in 1 and a facial conﬁg-uration was found in 2, 3, and 4.

Selected bond distances and bond angles of complex 1 are shown in Table 3. The two carbonyls, C(3)–O(1)

and C(5)–O(3), which are trans to each other, have C@O

bond distances of 1.139(4) and 1.129(4) A, respectively, while the other two carbonyls, C(4)–O(2) and C(6)– O(4), which are trans to the cyanotrihydroborate ligand,

have C@O bond distances of 1.152(4) and 1.155(4) A,

respectively. The longer bond distances of C(4)–O(2) and C(6)–O(4) indicate a stronger p back bonding ability of these two carbonyls. This eﬀect is also reﬂected on the Cr–C distances. While the Cr(1)–C(3) and Cr(1)–

C(5) have bond distances of 1.905(4) and 1.884(4) A,

respectively, the two carbonyls which are trans to the cyanotrihydroborate ligand have the Cr–C bond

Fig. 1. Molecular structures of the anion in [N(CH3)4]2[Cr(CO)4

(NCBH3)2], (1) showing 50% probability thermal ellipsoids.

Fig. 2. Molecular structures of the anion in [N(CH3)4]3[Cr(CO)3

(NCBH3)3] DME, (2) showing 50% probability thermal ellipsoids.

Table 1

Selected infrared data of compounds 1, 2, 3, and 4

mBH mCN mCO(cm1) [N(CH3)4]2[Cr(CO)4(NCBH3)2] 2342(m) 2286(w, sh) 1129(m) 2227(w) 2197(w) 1898(vs, br) 1869(vs) 1858(vs) 1803(vs) [N(CH3)4]3[Cr(CO)3(NCBH3)3] DME 2342(m) 2295(w, sh) 1130(m) 2229(vw) 2194(vw) 1897(vs) 1750(vs, br) [N(CH3)4]3[Mo(CO)3(NCBH3)3] DME 2345(m) 2291(w, sh) 1126(m) 2229(w) 2190(w) 1896(vs) 1764(vs, br) [N(CH3)4]3[W(CO)3(NCBH3)3] DME 2344(m) 2289(w, sh) 1126(m) 2228(vw) 2189(vw) 1884(vs) 1741(vs, br) [N(CH3)4][NCBH3] 2339(s) 2301(s, sh) 1132(m) 2232(w) 2172(s) Na[BH3CN] 2367(s) 2347(s,sh) 1135(m) 2244(w) 2185(s)

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distances of 1.826(4) and 1.827(4) A . The CBN bond

distances are 1.129(4) and 1.137(4) A, and the Cr–N–C

angles are 175.2(3)° and 176.5(3)° in 1. These bond distances and bond angles are comparable to those found in neutral, N bound cyanotrihydroborate com-plexes [4], where the bond distances occur in the range of

1.126(4)–1.16(2) A and the M–N–C angles occur in the

range of 164(1)–179(1)°. These results indicate the met-al-cyanotrihydroborate interaction in the anionic com-plex is similar to that in the neutral comcom-plexes.

Selected bond distances and bond angles of complex 2 are shown in Table 4. An apparent p back bonding eﬀect of 2 is observed in the comparison with that of the cya-notrihydroborate disubstituted complex 1. Each car-bonyl in 2 is trans to a cyanotrihydroborate ligand

having C@O bond distances of 1.158(4), 1.163(5) and

1.179(5) A, and having Cr–C bond distances of 1.810(4), 1.820(4), and 1.824(4) A . These C@O bond distances are longer, and the Cr–C bond distances are shorter than those observed in 1, due to the weak p back bonding ability of the cyanotrihydroborate ligand in 2. The CBN bond distances in 2 are 1.138(4), 1.145(5), and 1.151(5)

A, and the Cr–N–C angles are 172.6(3)°, 177.2(3)°. The selected bond distances and bond angles of complexes 3 and 4 are shown in Tables 5 and 6. The

C@O bond distances are 1.175(4), 1.176(4) and 1.182(4)

A in 3 and 1.161(7), 1.163(8) and 1.170(7) A in 4. These

C@O bond distances are longer than those found in the

cyanotrihydroborate disubstituted complex 1. The CBN

bond distances are 1.137(4), 1.144(4), and 1.144(4) A,

and the Mo–N–C angles are 171.9(3)°, 177.1(3)°, and 178.6(3)° in 3, and the CBN bond distances are 1.125(7),

1.132(7), and 1.141(7) A, and the W–N–C angles are

174.2(5)°, 175.9(5)°, and 179.6(5)° in 4.

3. Experimental 3.1. General procedures

All manipulations were carried out on a standard high vacuum line or in a drybox under an atmosphere of nitrogen. 1, 2-Dimethoxyethane (DME) and THF were dried over sodium/benzophenone and freshly distilled

prior to use. Acetonitrile was dried over P2O5 and

freshly distilled prior to use. Cr(CO)6, Mo(CO)6,

and W(CO)6were purchased from Strem Chemicals and

used as received. Sodium cyanotrihydroborate was

purchased from Acros, and [N(CH3)4]Cl was purchased

from Aldrich. [N(CH3)4][NCBH3] was prepared from

Table 2

Crystallographic data for [N(CH3)4]2[Cr(CO)4(NCBH3)2 (1), [N(CH3)4]3[Cr(CO)3(NCBH3)3] DME (2), [N(CH3)4]3[Mo(CO)3(NCBH3)3] DME

(3), and [N(CH3)4]3[W(CO)3(NCBH3)3] DME (4)

Empirical formula C14H30B2N4O4Cr C22H55B3N6O5Cr C22H55B3N6O5Mo C22H55B3N6O5W

Formula weight 392.04 568.15 612.09 699.93

Temperature (K) 293(2) 200(2) 150(1) 293(2)

Crystal system monoclinic monoclinic monoclinic monoclinic Space group P21=c P21=n P21=n P21=c a (A) 12.2737(8) 12.7774(10) 12.8302(1) 12.8621(6) b (A) 13.7533(9) A 16.9412(10) 16.9505(2) 17.2107(8) c (A) 14.4740(10) A 17.3407(10) 17.3100(2) 17.5164(8) b(°) 107.2590(10) 109.981(10) 110.3705(7) 109.1530(10) V (A3₎ _2333.3(3) _3527.7(4) _3529.12(6) _3662.9(3) Z 4 4 4 4 qcalc(g/cm3) 1.116 1.070 1.152 1.253 Crystal size (mm) 0.29 0.48 0.62 0.42 0.23 0.12 0.40 0.30 0.17 0.54 0.57 0.72 Radiation (k, A) Mo-Ka (0.71073) Mo-Ka (0.71073) Mo-Ka (0.71073) Mo-Ka (0.71073) 2h limits (°) 2.09–26.01 2.40–25.03 1.72–27.50 1.71–26.02 Index ranges 13 6 h 6 15 15 6 h 6 15 16 6 h 6 16 15 6 h 6 12 16 6 k 6 16 20 6 k 6 20 18 6 k 6 22 21 6 k 6 17 17 6 l 6 14 20 6 l 6 20 21 6 l 6 22 21 6 l 6 21 Reﬂections collected 12,946 60,112 22,918 20,406 Unique reﬂections 4560 6224 8089 7197

Unique reﬂections½I > 2:0rðIÞ] 832 1232 1304 1396

Completeness to h (%) 99.5 99.9 99.8 99.8 l(mm1₎ _0.511 _0.359 _0.407 _3.187 Data/restraints/parameters 4560/0/250 6224/0/370 8089/0/332 7197/0/334 R1a½I > 2:0rðIÞ 0.0513 0.0727 0.0529 0.0389 wR2b(all data) 0.1642 0.2316 0.1574 0.1255 Rint 0.0377 0.0434 0.0425 0.0398 Goodness-of-ﬁt on F2 _0.955 _1.099 _1.104 _0.825 a R1¼PjjFoj jFcjj=PjFoj. b_wR 2¼nPðFo2 Fc2Þ 2 P wðF2 oÞ 2 . o1=2 .

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the reaction of Na[BH3CN] with [N(CH3)4]Cl.

Cr(CO)3(CH3CN)3 and W(CO)3(CH3CN)3 were

pre-pared by literature procedures [6]. Elemental analyses were recorded on a Hitachi 270-30 spectrometer. Proton spectra (d(TMS) 0.00 ppm) were recorded on a Varian Mercury 200 spectrometer operating at 199.975 MHz or recorded on a Varian Unity Inova 600 spectrometer

operating at 599.948 MHz. 11_{B spectra (externally}

ref-erenced to BF3 OEt2(d 0.00 ppm)) were recorded on a

Varian Unity Inova 600 operating at 192.481 MHz. Infrared spectra were recorded on a Jasco FT/IR-460

Plus spectrometer with 2 cm1 resolution.

3.2. X-ray structure determination

Suitable crystals of 1, 3 and 4 were mounted and sealed inside glass capillaries under a nitrogen atmo-sphere. A single crystal of 2 was mounted on the tip of a glass ﬁber coated with fomblin oil (a perﬂuoropolye-ther). Crystallographic data collections of 1 and 4 were

carried out on a Bruker AXS SMART 1000 diﬀrac-tometer with graphite monochromated Mo Ka

radia-tion (k¼ 0:71073 A) at 293(2) K. The absorption

correction was based on the symmetry-equivalent re-flections and was applied to the data using SADABS program [11]. Cell parameters were retrieved and refined using a SHELXTL PLUS package [12]. Crystallo-graphic data collections of 2 and 3 were carried out on a Nonius KappaCCD diffractometer with graphite

monochromated Mo Ka radiation (k¼ 0:71073 A)

at 200(2) K (2) and 150(1) K (3). An empirical absorp-tion was based on the symmetry-equivalent reflecabsorp-tions and was applied to the data using the SORTAV pro-gram [13]. Cell parameters were retrieved and refined using DENZO-SMN software [14] on all reflections. Data reduction was performed with the DENZO-SMN software [14]. Structure analysis was made by using the SHELXTL program on a personal computer. The structure was solved using the SHELXS-97 program [15]

Table 3

Selected bond distances (A) and bond angles (deg) for [N(CH3)4]2

[Cr(CO)4(NCBH3)2], (1) Cr(1)–C(6) 1.826(4) Cr(1)–C(4) 1.827(4) Cr(1)–C(5) 1.884(4) Cr(1)–C(3) 1.905(4) Cr(1)–N(1) 2.089(3) Cr(1)–N(2) 2.091(3) C(3)–O(1) 1.139(4) C(4)–O(2) 1.152(4) C(5)–O(3) 1.129(4) C(6)–O(4) 1.155(4) N(1)–C(1) 1.137(4) N(2)–C(2) 1.129(4) B(1)–C(1) 1.582(6) B(2)–C(2) 1.588(6) C(6)–Cr(1)–C(4) 89.74(15) C(6)–Cr(1)–C(5) 86.96(16) C(4)–Cr(1)–C(5) 86.53(16) C(6)–Cr(1)–C(3) 90.24(16) C(4)–Cr(1)–C(3) 87.10(16) C(5)–Cr(1)–C(3) 173.05(15) C(6)–Cr(1)–N(1) 173.23(13) C(4)–Cr(1)–N(1) 96.46(13) C(5)–Cr(1)–N(1) 90.67(14) C(3)–Cr(1)–N(1) 92.80(14) C(6)–Cr(1)–N(2) 89.74(12) C(4)–Cr(1)–N(2) 178.83(13) C(5)–Cr(1)–N(2) 94.48(13) C(3)–Cr(1)–N(2) 91.86(14) N(1)–Cr(1)–N(2) 84.11(10) C(1)–N(1)–Cr(1) 175.2(3) C(2)–N(2)–Cr(1) 176.5(3) N(1)–C(1)–B(1) 177.8(4) N(2)–C(2)–B(2) 179.4(4) O(1)–C(3)–Cr(1) 173.6(4) O(2)–C(4)–Cr(1) 176.7(3) O(3)–C(5)–Cr(1) 173.4(3) O(4)–C(6)–Cr(1) 177.5(3) Table 4

Selected bond distances (A) and bond angles (deg) for[N(CH3)4]3

[Cr(CO)3(NCBH3)3] DME, (2) Cr(1)–C(4) 1.810(4) Cr(1)–C(5) 1.820(4) Cr(1)–C(6) 1.824(4) Cr(1)–N(1) 2.098(3) Cr(1)–N(2) 2.109(3) Cr(1)–N(3) 2.112(3) C(4)–O(4) 1.179(5) C(5)–O(5) 1.158(4) O(6)–C(6) 1.163(5) N(1)–C(1) 1.145(5) N(2)–C(2) 1.138(4) N(3)–C(3) 1.151(5) C(1)–B(1) 1.590(7) C(3)–B(3) 1.581(6) C(2)–B(2) 1.590(6) C(4)–Cr(1)–C(5) 85.25(16) C(4)–Cr(1)–C(6) 86.80(17) C(5)–Cr(1)–C(6) 85.94(16) C(4)–Cr(1)–N(1) 94.22(14) C(5)–Cr(1)–N(1) 95.05(14) C(6)–Cr(1)–N(1) 178.63(14) C(4)–Cr(1)–N(2) 96.44(13) C(5)–Cr(1)–N(2) 178.11(14) C(6)–Cr(1)–N(2) 94.98(14) C(4)–Cr(1)–N(3) 177.34(13) C(5)–Cr(1)–N(3) 92.49(14) C(6)–Cr(1)–N(3) 94.44(14) N(1)–Cr(1)–N(2) 84.01(11) N(1)–Cr(1)–N(3) 84.57(11) N(2)–Cr(1)–N(3) 85.80(11) O(4)–C(4)–Cr(1) 174.9(3) O(5)–C(5)–Cr(1) 175.8(4) O(6)–C(6)–Cr(1) 176.3(3) C(1)–N(1)–Cr(1) 177.2(3) C(3)–N(3)–Cr(1) 172.6(3) C(2)–N(2)–Cr(1) 178.5(3) N(1)–C(1)–B(1) 178.2(4) N(2)–C(2)–B(2) 179.2(4) N(3)–C(3)–B(3) 179.0(4)

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and reﬁned using SHELXL-97 program [16] by full-matrix least-squares on F2_{values. For each structure, all}

nonhydrogen atoms were located and refined aniso-tropically. The hydrogen atoms on the boron atoms in 1 and 2 were located and refined isotropically and other hydrogen atoms were fixed at calculated positions and refined using a riding mode. Crystallographic data of 1, 2, 3, and 4 are summarized in Tables 2–6. For complexes 2, 3 and 4, one DME molecule was also found in the crystal lattice.

3.3. Preparations of complexes

3.3.1. [N(CH3)4]2[Cr(CO)4(NCBH3)2] (1)

Cr(CO)6(660 mg, 3.0 mmol), NaBH3CN (379 mg, 6.0

mmoL), and DME (20 ml) were placed in a 100 mL flask. After degasing, the flask was heated to reflux. During the process, the solution changes color from colorless to orange gradually. The carbon monoxide

formed in this reaction was removed. After heating for 30 h, the solution was cooled to room temperature and

660 mg (6.0 mmol) of (CH3)4NCl was added to the

system. The solution changed color to light yellow after adding (CH3)4NCl. After stirring for 5 h, a yellow

so-lution with yellow solids was formed. Sodium chloride was separated from the system through extraction of the product by DME solvent. A 729 mg (62% yield) of [N(CH3)4]2[Cr(CO)4(NCBH3)2] was isolated after

removal of the solvent. Yellow crystals of [N(CH3)4]2

[Cr(CO)4(NCBH3)2] were isolated from CH3CN/THF

solution at room temperature. 11_{B NMR (CH}

3CN) d¼ 41:5 ppm (q, JB–H¼ 89 Hz). 1H NMR (d3 -aceto-nitrile): d 3.09 (s, N(CH3)4), 0.30 ppm (br, q, BH3). IR(KBr): 3304(vw), 2962(vw), 2342(m), 2286(w, sh), 2227(w), 2197(w), 2171(vw), 2070(w), 1898(vs, br), 1869(vs), 1858(vs), 1803(vs), 1484(m), 1449(w), 1420(w), 1287(vw), 1129(m), 1019(vw), 949(m), 864(vw), 738(vw),

655(s), 557(w), 432(m) cm1_. _Anal. _Calcd _for

Table 5

Selected bond distances (A) and bond angles (deg) for [N(CH3)4]3[Mo(CO)3(NCBH3)3] DME, (3) Mo–C(2) 1.929(3) Mo–C(3) 1.929(4) Mo–C(1) 1.937(4) Mo–N(3) 2.236(3) Mo–N(2) 2.245(3) Mo–N(1) 2.246(3) C(1)–O(1) 1.175(4) C(2)–O(2) 1.176(4) C(3)–O(3) 1.182(4) N(1)–C(4) 1.144(4) N(2)–C(5) 1.137(4) N(3)–C(6) 1.144(4) C(4)–B(1) 1.584(5) C(5)–B(2) 1.581(5) C(6)–B(3) 1.589(6) C(2)–Mo–C(3) 84.88(14) C(2)–Mo–C(1) 85.81(14) C(3)–Mo–C(1) 87.09(15) C(2)–Mo–N(3) 95.95(13) C(3)–Mo–N(3) 94.94(13) C(1)–Mo–N(3) 177.41(12) C(2)–Mo–N(2) 178.01(13) C(3)–Mo–N(2) 96.70(12) C(1)–Mo–N(2) 95.47(12) C(2)–Mo–N(1) 93.58(12) C(3)–Mo–N(1) 177.00(12) C(1)–Mo–N(1) 95.37(12) N(3)–Mo–N(2) 82.71(10) N(3)–Mo–N(1) 82.65(10) N(2)–Mo–N(1) 84.79(10) C(4)–N(1)–Mo 171.9(3) C(5)–N(2)–Mo 178.6(3) C(6)–N(3)–Mo 177.1(3) O(1)–C(1)–Mo 178.4(3) O(2)–C(2)–Mo 179.0(3) O(3)–C(3)–Mo 176.6(3) N(1)–C(4)–B(1) 178.3(3) N(2)–C(5)–B(2) 179.3(4) N(3)–C(6)–B(3) 178.9(4) Table 6

Selected bond distances (A) and bond angles (deg) for [N(CH3)4]3[W(CO)3(NCBH3)3] DME, (4) W–C(5) 1.930(7) W–C(6) 1.929(7) W–C(4) 1.942(7) W–N(3) 2.210(5) W–N(1) 2.211(5) W–N(2) 2.223(5) C(4)–O(1) 1.161(7) C(5)–O(2) 1.170(7) C(6)– O(3) 1.163(8) N(1)–C(1) 1.125(7) N(2)–C(2) 1.132(7) N(3)–C(3) 1.141(7) B(1)–C(1) 1.612(11) B(2)–C(2) 1.601(11) B(3)–C(3) 1.604(9) C(5)–W–C(6) 86.2(3) C(5)–W–C(4) 88.3(3) C(6)–W–C(4) 86.3(3) C(5)–W–N(3) 96.5(2) C(6)–W–N(3) 177.0(3) C(4)–W–N(3) 95.0(2) C(5)–W–N(1) 94.8(2) C(6)–W–N(1) 96.0(3) C(4)–W–N(1) 176.2(2) N(3)–W–N(1) 82.51(18) C(5)–W–N(2) 176.8(2) C(6)–W–N(2) 93.3(3) C(4)–W–N(2) 94.9(2) N(3)–W–N(2) 83.90(17) N(1)–W–N(2) 82.00(18) C(1)–N(1)–W 175.9(5) C(2)–N(2)–W 174.2(5) C(3)–N(3)–W 179.6(5) N(1)–C(1)–B(1) 179.0(8) N(2)–C(2)–B(2) 177.8(7) N(3)–C(3)–B(3) 178.9(7) O(1)–C(4)–W 177.9(6) O(2)–C(5)–W 177.1(6) O(3)–C(6)–W 179.3(7)

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C14H30B2N4O4Cr: N, 14.29; C, 42.89; H, 7.71; Found:

N, 14.34; C, 42.79; H, 7.80%.

3.3.2. [N(CH3)4]3[Cr(CO)3(NCBH3)3] DME (2)

Cr(CO)3(CH3CN)3 (260 mg, 1.0 mmol) and

NaBH3CN (190 mg, 3.0 mmol) were placed in a 50 mL

ﬂask, and the ﬂask was evacuated. About 20 mL of

acetonitrile was transferred into the ﬂask at)78 °C. The

ﬂask was warmed to room temperature and heated to reﬂux for 3 h. The acetonitrile was removed under

vacuum and 329 mg (3.0 mmol) of (CH3)4NCl was

charged to the flask. After degasing, about 20 mL of DME was transferred into the flask and the system was stirred at room temperature for 5 h. The yellow solution was separated from the yellow solids through filtration and the yellow solids were washed with 10 mL of DME twice. The filtrate was discarded and the yellow solids were extracted with acetonitrile. After removal of the solvent, the yellow solids were redissolved in a

CH3CN/DME mixed solvent for crystallization. Yellow

crystals (397 mg, 70% yield) of [N(CH3)4]3[Cr(CO)3

(NCBH3)3] DME were obtained.11B NMR (CH3CN) d

)42.9 ppm (JB–H¼ 89 Hz).1H NMR (d3-acetonitrile): d 3.45 (s, DME), 3.39 (s, DME), 3.11 (s,N(CH3)4), 0.36 ppm (br, q, BH3). IR(KBr): 3035(vw), 2963(vw), 2342(m), 2295(w, sh), 2229(vw), 2194(vw), 1897(vs), 1750(vs, br), 1485(m), 1448(vw), 1418(vw), 1261(vw), 1130(m), 1021(w), 949(m), 868(vw), 801(vw), 653(vw),

551(vw) cm1_{. Anal. Calcd for C}

22H55B3N6O5Cr: N,

14.79; C, 46.51; H, 9.76; Found: N, 14.61; C, 46.11; H, 9.69%.

3.3.3. [N(CH3)4]3[Mo(CO)3(NCBH3)3] DME (3)

A 262 mg (0.99 mmol) quantity of Mo(CO)6, 190.5

mg (3.0 mmol) of NaBH3CN, and 20 mL of DME were

placed in a 100 mL flask. The system was evacuated and heated to reflux. The carbon monoxide formed was re-moved. During the process, the solution changed color to orange gradually and eventually it turned cloudy. The reaction was quenched after refluxing for three days and a two-layer phase was observed at room temperature. The lower layer is orange in color and the upper layer is light yellow in color. A 658 mg (6.0 mmol) quantity of (CH3)4NCl were added to this system. After stirring for

5 h, the solvent was removed and the product was ex-tracted with acetonitrile. Brown solids were obtained after removal of the solvent. The resulting brown solids

were redissolved in a CH3CN/DME mixed solvent for

crystallization. A 448 mg (74% yield) of light yellow

crystals were obtained. 11_{B NMR (d}

3-acetonitrile):

d¼ 42:1 ppm (q, JB–H¼ 89 Hz). 1H NMR (d3

-aceto-nitrile): d¼ 3.45 (s, DME), 3.39 (s, DME), 3.13 (s,

N(CH3)4), 0.36 ppm (br, q, BH3). IR(KBr): 3487(w),

3034(w), 2957(vw), 2345(m), 2291(w, sh), 2229(w), 2190(w), 1896(vs), 1764(vs, br), 1484(s), 1448(w), 1416(w), 1286(vw), 1126(m), 1057(vw), 948(s), 864(w),

638(vw), 532(vw), 486(vw) cm1. Anal. Calcd for

C22H55B3N6O5Mo: N, 13.73; C, 43.17; H, 9.06; Found:

N, 13.34; C, 41.74; H, 8.99%.

3.3.4. [N(CH3)4]3[W(CO)3(NCBH3)3] DME (4)

Method 1. W(CO)6 (347.5 mg, 0.99 mmol),

NaBH3CN (196.0 mg, 3.1 mmol) and 20 mL of DME

were placed in a 100 mL flask. The flask was evacu-ated and heevacu-ated to reflux. The solution changed color to yellow gradually, and the carbon monoxide formed was removed. The solvent was removed after refluxing for four days. A 329 mg (3 mmol) quantity of

(CH3)4NCl was charged to the ﬂask and the solution

was stirred at room temperature for 6 h. The DME was removed and the resulting products were ex-tracted with acetonitrile. After removal of the solvent, yellow solids were obtained and they were dissolved in

a CH3CN/DME mixed solvent for crystallization. A

76 mg (10.8% yield) of [N(CH3)4]3[W(CO)3(NCBH3)3]

DME was isolated.

Method 2. W(CO)3(CH3CN)3 (391 mg, 1.0 mmol)

and NaBH3CN (189 mg, 3.0 mmol) were placed in a 50

mL ﬂask, and the ﬂask was evacuated. About 20 mL of

acetonitrile was transferred into the ﬂask at)78 °C. The

system was heated to reﬂux for 3 h, then the acetonitrile

was removed and 330 mg (3.0 mmol) of (CH3)4NCl was

charged to the flask. After degassing, about 20 mL of DME was transferred into the flask. After stirring at room temperature for 6 h, The DME solvent was re-moved and a 20 mL portion of acetonitrile was trans-ferred into the flask. The NaCl was removed by filtration and the acetonitrile was evacuated. The

re-sulting yellow solid was redissolved in a CH3CN/DME

mixed solvent for crystallization. A 532 mg (76% yield) of [N(CH3)4]3[W(CO)3 NCBH3)3] DME was obtained.

11_{B NMR (CH} 3CN): d 41:5 ppm (q, JB–H¼ 89 Hz). 1_{H NMR (d} 3-acetonitrile): d¼ 3:44 (s, DME), 3.27 (s, DME), 3.11 (s, N(CH3)4), 0.37 ppm (br, q, BH3). IR(KBr): 3445(w), 2909 (vw), 2344(m), 2289(w, sh), 2228(w), 2189(w), 1884(vs), 1741(vs, br), 1483(m), 1126(m), 1100(w), 1029(vw), 949 (m) cm1_{. Anal. Calcd} for C22H55B3N6O5 W: N, 12.00; C, 37.75; H, 7.92; Found: N, 12.08; C, 36.74; H, 7.81%. 4. Supplementary material

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 217842 (compound 1), 217843 (compound 2), 217844 (compound 3), 217845 (com-pound 4), Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:

+44-1223-336033; e-mail:[email protected] www:http://

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Acknowledgements

We thank Dr. Shengming Liu (The Ohio State Uni-versity) for the X-ray crystallography of complexes (2). This work was supported by the National Science Council of the ROC through Grant NSC 91-2113-M-259-010.

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