Lanthanide(III)-bis(cyclopropylethinylamidinates): Synthesis, structure, and catalytic activity

Journal of Organometallic Chemistry 785 (2015) 1e10

Contents lists available at ScienceDirect

Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem

Lanthanide(III)-bis(cyclopropylethinylamidinates): Synthesis, structure, and catalytic activity Farid M. Sroor a, Cristian G. Hrib a, Liane Hilfert a, Peter G. Jones b, Frank T. Edelmann a, * a b

€t Magdeburg, Universita €tsplatz 2, D-39106 Magdeburg, Germany Chemisches Institut der Otto-von-Guericke-Universita Institut für Anorganische und Analytische Chemie der TU Braunschweig, Hagenring 30, 38106 Braunschweig, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 November 2014 Received in revised form 26 January 2015 Accepted 29 January 2015 Available online 28 February 2015

Reactions of anhydrous lanthanide trichlorides, LnCl3 (Ln ¼ Ce, Nd, Ho), with 2 equiv. of lithiumcyclopropylethinylamidinates, Li[c-C3H5eC^CeC(NR)2] (1a: R ¼ iPr, 1b: R ¼ cyclohexyl (Cy)), afforded a series of new lanthanide-bis(cyclopropylethinylamidinates). In the case of cerium and neodymium, the chloro-bridged dimers [{c-C3H5eC^CeC(NR)2}2Ln(m-Cl)(THF)]2 (2a: Ln ¼ Ce, R ¼ iPr; 2b: Ln ¼ Ce, R ¼ Cy; 2c: Ln ¼ Nd, R ¼ Cy) were isolated, whereas the smaller holmium afforded the “ate” complex [cC3H5eC^CeC(NCy)2]2Ho(m-Cl)2Li(THF) (OEt2) (3). An initial study showed that these complexes effectively catalyze the addition of aniline derivatives to carbodiimides to give N-arylguanidines. The new complexes 2aec and 3 as well as the N-arylguanidines o-C6H4(NH2)[-N]C(NHiPr)2] (8) and p-C6H4Cl [-N]C(NHiPr)2] (9) have been structurally characterized by X-ray diffraction. © 2015 Elsevier B.V. All rights reserved.

Keywords: Amidinates Cerium Neodymium Holmium Catalysis Crystal structure

Introduction In organolanthanide chemistry, the investigation of new spectator ligands which satisfy the coordination requirements of the large lanthanide ions continues to be a hot topic. A highly successful approach in this area is the use of monoanionic amidinate and guanidinate ligands of the types [RC(NR0 )2] (R ¼ H, alkyl, aryl; R0 ¼ alkyl, cycloalkyl, aryl, SiMe3) and [R2NC(NR0 )2] (R ¼ alkyl, SiMe3; R0 ¼ alkyl, cycloalkyl, aryl, SiMe3). Both of these chelating anions have been demonstrated to be steric cyclopentadienyl equivalents [1]. Mono-, di- and trisubstituted lanthanide amidinate and guanidinate complexes are all readily accessible. Various rareearth metal amidinates and guanidinates have been reported to be very efficient homogeneous catalysts e.g. for ring-opening polymerization reactions of lactones or the guanylation of amines. Homoleptic alkyl-substituted lanthanide tris(amidinates) and tris(guanidinates) have been found to be highly volatile and useful as promising precursors for ALD (¼ atomic layer deposition) and MOCVD (¼ metal-organic chemical vapor deposition) processes in materials science, e.g. for the production of lanthanide oxide (Ln2O3) or nitride (LnN) thin layers. Overall, the past ca. 25 years

* Corresponding author. Tel.: þ49 391 6758327; fax: þ49 391 6712933. E-mail address: [email protected] (F.T. Edelmann). http://dx.doi.org/10.1016/j.jorganchem.2015.01.034 0022-328X/© 2015 Elsevier B.V. All rights reserved.

have witnessed a remarkable transformation of lanthanide amidinates and guanidinates from laboratory curiosities to highly active homogeneous catalysts and valuable precursors in materials science [2]. A particularly useful variation of the amidinate ligand theme is the attachment of alkinyl groups at the central carbon atom to afford alkinylamidines, RC^CeC(¼NR0 ) (NHR0 ) [3]. In organic synthesis, alkinylamidines are well known as valuable reagents for the preparation of various heterocycles [4], and certain alkinylamidines have been found to be useful antitussives [5]. More recently, alkinylamidines have attracted considerable attention due to their diverse applications in biological and pharmacological systems [6]. Transition metal and lanthanide alkinylamidinate complexes have been demonstrated to be efficient and versatile catalysts e.g. for CeC and CeN bond formation, the addition of CeH, NeH and PeH bonds to carbodiimides as well as ε-caprolactone polymerization [3f,7]. In the course of our ongoing investigation of lanthanide amidinates we reasoned that alkinylamidinates derived from cyclopropylacetylene would represent an interesting and potentially useful addition to the current library of amidinate ligands. Following a report that lithium-trimethylsilylethinylamidinates are readily available from the reaction between either N,N0 -diisopropylcarbodiimide or N,N0 -dicyclohexylcarbodiimide and lithiumtrimethylsilylacetylide in diethyl ether [8], we recently described the synthesis and full characterization of a series of lithium-

2

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10

cyclopropylethinylamidinates [9]. These precursors are readily available on a large scale using commercially available starting materials. Here we report the first rare-earth metal complexes comprising cyclopropylethinylamidinate ligands and their use as guanylation catalysts. Result and discussion The starting lithium-cyclopropylethinylamidinates 1a and 1b were prepared in a straightforward manner according to Scheme 1 by in situ-deprotonation of commercially available cyclopropylacetylene followed by treatment with either N,N0 -diisopropylcarbodiimide or N,N0 -dicyclohexylcarbodiimide according to the published procedure [9]. For the following reactions with lanthanide trichlorides, the reagents 1a and 1b were conveniently prepared in THF solution and used in situ. Subsequent reactions with anhydrous CeCl3 and NdCl3 were carried out in a 1:2 molar ratio in THF solutions at 65  C (2 h) followed by stirring at ambient temperature. Evaporation of the volatiles and subsequent recrystallization of the solid residues from npentane afforded the chloro-functional lanthanide(III) bis(cyclopropylethinylamidinate) complexes 2aec in moderate (2a: 62%, 2b: 55%) to good (2c: 85%) yields. From a similar reaction of anhydrous NdCl3 with 2 equiv. of 1a the chloro-bridged bis(amidinate) complex [{c-C3H5eC^CeC(NiPr)2}2Nd(m-Cl) (THF)]2 could not be isolated as a pure material. The two cerium derivatives 2a and 2b were isolated in the form of exceedingly air- and moisture-sensitive, bright yellow, needle-like crystals, whereas the neodymium complex 2c crystallizes in the form of dark green needles. The products are well soluble in THF, diethyl ether, toluene and n-pentane. X-Ray diffraction studies (vide infra) revealed that all three compounds are chloro-bridged dimers, so that the reactions can be formulated as illustrated in Scheme 2. All three compounds were characterized by their NMR (1H, 13C) and IR spectra as well as elemental analyses. Despite the paramagnetic nature of the Ce3þ and Nd3þ ions, meaningful NMR spectra could be obtained for all three compounds. The data were in good agreement with the formation of lanthanide(III) bis(cyclopropylethinylamidinates) as their mono-THF adducts, although the 1 H NMR signals were significantly broadened. Bis(amidinato) and bis(guanidinato) lanthanide chlorides are known to occur in three different types [2]: (a) THF-solvated monomers L2LnCl(THF) [10]; (b) “ate” complexes such as L2Ln(m-Cl)2Li(THF)2 [11]; and (c) chlorobridged dimers [L2Ln(m-Cl)]2 (L ¼ amidinate or guanidinate anion) [12,13], with the dimers being less common than the “ate” complexes. The 1H NMR data of 2aec provided little or no indication of the presence of any of these three structural types in THF-d8 solution. The IR spectra of 2aec were found to be almost superimposable. IR bands attributable to the C]N stretching vibrations of the NeCeN units appear at around 1610 cm1, whereas very strong bands at 2221e2227 cm1 can be assigned to the C^C vibrations. Bright yellow (2a,b) or dark green (2c), needle-like single crystals suitable for X-ray diffraction were obtained by slow cooling of

Scheme 1. Preparation of the lithium-cyclopropylethinylamidinates 1a and 1b.

saturated solutions in n-pentane to 30  C. Single-crystal X-ray diffraction studies of 2aec clearly established the presence of chloro-bridged dimers of the type [{c-C3H5eC^CeC(NR)2}2Ln(mCl) (THF)]2 (2a: Ln ¼ Ce, R ¼ iPr; 2b: Ln ¼ Ce, R ¼ Cy; 2c: Ln ¼ Nd, R ¼ Cy). The molecular structures of 2a and 2b as one iPr- and one Cy-substituted derivative are depicted in Figs. 1 and 2; the crystal and structural refinement data are summarized in Table 1. Complete structural data of 2aec can be found in the Supplementary material. Since the molecular structures of all three compounds 2aec are very similar, only the structures of 2a and 2b will be discussed here as typical representatives. The new compounds are centrosymmetric dimers of the type [L2Ln(m-Cl) (THF)]2 with a planar fourmembered Ce2Cl2 ring as the central structural unit. With angles of 106.59(3) (CeeCleCe) and 73.41(3) the Ce2Cl2 moiety is rhomb-shaped. The cyclopropylethinylamidinate ligands adopt the “classical” k2N,N0 -chelating coordination mode. As expected, the chloro ligands act as bridging ligands in all cases. The coordination sphere around the large Ce3þ and Nd3þ ions still leaves room for an additional THF ligand. Thus the formal coordination number around the metal atoms is 7. In this respect the cyclopropylethinylamidinates differ from the related dimeric yttrium, ytterbium, and lutetium bis(guanidinato) complexes such as [{(Me3Si)2NC(NiPr)2}2Y(m-Cl)]2 [12a] and [{(Me3Si)2NC(NCy)2}2Ln(m-H)]2 [14]. In these compounds the steric bulk of the supporting guanidinate ligands prevents the formation of THF adducts, resulting in 6-coordinate lanthanide ions. Thus it can be assumed that the steric demand of the cyclopropylethinylamidinate anions is significantly lower than that of the N,N-bis(trimethylsilyl)-substituted guanidinate anion [(Me3Si)2NC(NCy)2]. In rare-earth metal complexes, amidinate and guanidinate ligands generally have small N-M-N bite angles. In 2a, these angles are 54.14(9) (N(1)-Ce-N(2)) and 53.93(9) (N(3)Ce-N(4)) which is significantly smaller than the NeYeN angles reported for [{(Me3Si)2NC(NiPr)2}2Y(m-Cl)]2 (55.95(14) and 57.06(15) ) [12a]. Within the chelating NCN unit the CeN distances are nearly equal (average CeN ¼ 1.33 Å), indicating p-electron delocalization within the amidinate unit. This can be favorably compared with an average value of e.g. 1.34 Å which has been reported for [{(Me3Si)2NC(NiPr)2}2Y(m-Cl)]2 [12a]. The bond lengths of the triple bonds in the cyclopropylethinyl units are 1.189(5) Å (C(2)-C(3)) and 1.191(5) Å (C22)eC(23). The X-ray diffraction data for 2b show that the different substituents on the amidinate N atoms (iPr vs. Cy) have no significant influence on the structural parameters. Both the NeCeeN bite angles and the CeN distance in the amidinate moiety are virtually identical in both compounds. It seems clear that the introduction of the “slim” cyclopropylethinyl substituents at the central carbon atom of the amidinate NCN unit diminishes the steric bulk of the resulting amidinate anions, so that with large Ln3þ ions dimers are obtained, in which there is room for additional THF coordination. In contrast, “ate” complex formation was observed when the smaller holmium was employed in an analogous reaction of HoCl3 with 2 equiv. of 1b as illustrated in Scheme 3. This reaction afforded the bright yellow “ate” complex [cC3H5eC^CC(N-c-C6H11)2]2Ho(m-Cl)2Li(THF) (Et2O) (3) in high yield (83%) after recrystallization from diethyl ether. Due to the strongly paramagnetic nature of Ho3þ, it was impossible to obtain interpretable NMR spectra for 3. Only an X-ray diffraction study clearly established the presence of the “ate” complex [c-C3H5eC^CC(N-cC6H11)2]2Ho(m-Cl)2Li(THF)(Et2O) formed by addition of lithium chloride to monomeric [c-C3H5eC^CC(N-c-C6H11)2]2HoCl (Table 1 and Fig. 3). Surprisingly, the tetrahedral coordination sphere of Li is supplemented by both THF and diethyl ether, apparently as a result of the reaction having been carried out in THF and the product

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10

3

Scheme 2. Synthesis of [{c-C3H5eC^CeC(NR)2}2Ln(m-Cl) (THF)]2 (2a: Ln ¼ Ce, R ¼ iPr; 2b: Ln ¼ Ce, R ¼ Cy; 2c: Ln ¼ Nd, R ¼ Cy).

recrystallized from diethyl ether. Besides this peculiarity, the molecular structure of 3 is very similar to previously reported lanthanide(III) bis(amidinate) or bis(guanidinate) “ate” complexes such as [2,4,6-(CF3)3C6H2C(NSiMe3)2]2Nd(m-Cl)2Li(THF)2 or [(Me3Si)2NC(NPri)2]2Ln(m-Cl)2Li(THF)2 (Ln ¼ Nd, Yb, Lu) [1,2,11]. In the year 2002, lanthanide amidinates were first reported to exhibit promising catalytic activities [2]. It was discovered that homoleptic lanthanide tris(amidinates) are extremely active catalysts for the ring-opening polymerization (¼ ROP) of ε-caprolactone. By now, amidinate and guanidinate complexes of rare-earth elements have been established as highly useful catalysts especially for the polymerization of polar monomers [2,11,15]. Besides these already established polymerization reactions, new catalytic applications of lanthanide amidinates and guanidinates have emerged in recent years. One such reaction is the Ln-catalyzed guanylation of amines with carbodiimides (also known as hydroamination of carbodiimides or addition of amines to carbodiimides) to afford guanidines [7c,16]. In the case of rare-earth metals, several types of lanthanide catalyst precursors containing Ln-C, Ln-N, or Ln-O bonds have been successfully employed [7b,7d,17,18]. For example, tris(phenoxides) of the type Ln(OAr)3(THF)2 (Ln ¼ Y, Yb; Ar ¼ 2,6-tBu24-MeC6H2) have recently been reported to be efficient precatalysts for this guanylation reaction under mild conditions. In the course of

this study it was established that mixed-ligand bis(alkoxide)/ mono(guanidinate) complexes are the active species in this process. In fact, this was the first example of catalytic guanylation catalyzed by rare-earth metal complexes comprising Ln-O bonds [17k,l]. Heterobimetallic Li/Ln dianionic guanidinate complexes have also been established as highly efficient precatalysts for the catalytic addition of amines to carbodiimides to synthesize guanidines. Such complexes were found to effectively catalyze the addition of various primary and secondary amines as well as aromatic and aliphatic diamines to carbodiimides to give the corresponding monoguanidine and biguanidine derivatives under mild conditions (25e60  C). This method provided an efficient way for the synthesis of biguanidine compounds. Activities were shown to depend on the central Ln metals and ligands: La > Nd > Y for the metals and [(iPrN)2C(NC6H4Cl-p)]2 > [(iPrN)2C(NC6H5)]2 for the ligands were observed [17i]. High catalytic activity for the guanylation of both aromatic and secondary amines under mild conditions has also been reported for the simple lanthanide silylamides Ln[N(SiMe3)2]3 (Ln ¼ Y, Yb) and [(Me3Si)2N]3Ln(m-Cl)Li(THF)3 (Ln ¼ Y, La, Sm, Eu, Yb) [17c]. Most recently, it has been reported that half-sandwich bis(amidinato) rare-earth metal complexes of the type (C5Me4SiMe3) Ln[PhC(NAr)2]2 (Ln ¼ Y, Dy, Er, Lu; Ar ¼ Ph, 4-MeC6H4) exhibit excellent catalytic activity for the addition of amines to

Fig. 1. X-ray molecular structure of [{c-C3H5eC^CeC(NiPr)2}2Ce(m-Cl) (THF)]2 (2a). All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles ( ): CeeN1 2.516(3), CeeN2 2.456(3), CeeN3 2.515(3), CeeN4 2.491(3), CeeCl 2.833(11), Ce-ClA 2.922(12), CeeO 2.450(2), N1eC1 1.328(4), N2eC1 1.337(4), C1eC2 1.444(5), C2eC3 1.189(5), C3eC4 1.446(5), N3eC21 1.332(4), N4eC21 1.334(4), C21eC22 1.453(5), C22eC23 1.191(5), C23eC24 1.436(5); N1eCeeN2 54.14(9), N3eCeeN4 53.93(9), N1eC1eN2 116.20(3), N3eC21eN4 116.8(3), OeCeeCl 100.51(7), CeeCl-CeA 106.59(3), CleCe-ClA 73.41(3).

Fig. 2. X-ray molecular structure of [{ceC3H5eC^CeC(NCy)2}2Ce(m-Cl) (THF)]2 (2b). All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles ( ): CeeN1 2.582(17), CeeN2 2.487(17), CeeN3 2.519(17), CeeN4 2.461(17), CeeCl 2.845(5), Ce-ClA 2.9057(4), CeeO 2.552(15), N1eC1 1.333(3), N2eC1 1.335(3), C1eC2 1.459(3), C2eC3 1.195(3), C3eC4 1.446(3), N3eC21 1.332(3), N4eC21 1.331(3), C21eC22 1.456(3), C22eC23 1.187(4), C23eC24 1.506(9); N1eCeeN2 53.42(6), N3eCeeN4 54.05(6), N1eC1eN2 117.44(18), N3eC21eN4 116.44(18), OeCeeCl 105.65(4), CleCeClA 74.277(14), CeeCl-CeA 105.722(14).

4

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10

Table 1 Crystallographic data and structure refinement details for complexes 2a, 2b, 2c, 3, 8, and 9.

Empirical formula M Crystal size/mm3 Crystal system Space group a [Å] b [Å] c [Å] a [ ] b [ ] g [ ] Cell volume [Å] Z dcalc, [g cm3] m [cm3] F000 Index ranges

Reflns collected Data/restraints/ parameters GOF R1 (I > 2s(I)) wR2 (all data) largest diff. peak and hole [e Å3]

2a

2b

2c

3

8

9

C28H46CeClN4O 630.26 0.60  0.50  0.46 triclinic P-1 10.051(2) 11.371(2) 14.512(3) 82.71(3) 83.10(3) 73.88(3) 1574.3(5) 2 1.330 1.555 650 13  h  13 14  k  15 19  l  19 16986 7771/0/325

C80 H124 Ce2Cl2 N8 O2 1581.01 0.14  0.08  0.07 monoclinic P21/c 11.25710(10) 19.86380(10) 19.26360(10) 90 99.3860(10) 90 4249.84(5) 2 1.235 9.090 1652 14  h  12 24  k  24 24  l  24 146163 8784/0/422

C80H124Cl2N8Nd2O2 1589.25 0.60  0.43  0.37 monoclinic P21/c 11.290(2) 19.824(4) 19.209(4) 90 99.86(3) 90 4235.7(15) 2 1.246 1.321 1660 15  h  15 27  k  26 22  l  26 31182 11375/0/422

C44H72Cl2HoLiN4O2 931.83 0.46  0.38  0.22 monoclinic P21/n 10.473(2) 29.334(6) 15.009(3) 90 91.61(3) 90 4609.2(16) 4 1.343 1.870 1936 13  h  13 39  k  39 20  l  20 31110 11314/44/458

C13H22N4 234.35 0.16  0.12  0.08 monoclinic P21/c 13.9953(2) 11.12560(10) 8.57570(10) 90 96.2330(10) 90 1327.40(3) 4 1.173 0.565 512 17  h  17 13  k  13 10  l  10 44933 2759/0/171

C26H40Cl2N6 507.54 0.56  0.48  0.33 monoclinic P21/n 8.5543(17) 25.614(5) 13.077(3) 90 91.17(3) 90 2864.6(10) 4 1.177 0.251 1088 9  h  11 35  k  35 17  l  17 21231 7677/0/323

1.035 0.0404 0.1100 0.954, 1.967

1.039 0.0274 0.0709 0.595, 0.851

0.992 0.0341 0.0882 0.624, 2.455

0.858 0.0428 0.0902 0.941, 1.514

1.071 0.0362 0.0930 0.212, 0.221

1.036 0.0526 0.1381 0.897, - 0617

Scheme 3. Synthesis of [c-C3H5eC^CeC(NCy)2]2Ho(m-Cl)2Li(THF)(OEt2) (3).

Fig. 3. X-ray molecular structure of [c-C3H5eC^CeC(NCy)2]2Ho(m-Cl)2Li(THF)(OEt2) (3). All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles ( ): Ho-N1 2.328(4), Ho-N2 2.352(3), Ho-N3 2.331(4), Ho-N4 2.359(3), Ho-Cl1 2.6428(14), Ho-Cl2 2.6574(15), Cl1eLi 2.348(12), Cl2eLi 2.368(12), N1eC1 1.336(6), N2eC1 1.335(6), C1eC2 1.448(6), C2eC3 1.179(6), C3eC4 1.445(6), N3eC21 1.329(5), N4eC21 1.342(5), C21eC22 1.442(6), C22eC23 1.192(6), C23eC24 1.440(6), LieO1 1.880(12), LieO2 1.954(12); N1-Ho-N2 57.70(13), N3-Ho-N4 57.45(12), Cl1-Ho-Cl2 83.70(5), N1eC1eN2 115.4(4), N3eC21eN4 115.2(4), Cl1eLieCl2 97.2(4), LieCl1-Ho 89.6(3), LieCl2-Ho 88.8(3).

carbodiimides [19]. Thus, in the present study the catalytic activity of the title compounds for the addition of aniline derivatives to carbodiimides is reported. This would be a rare example (besides [(Me3Si)2N]3Ln(m-Cl)Li(THF)3 (Ln ¼ Y, La, Sm, Eu, Yb) [17c] of lanthanide chloro complexes being active precatalysts for such reactions. For an initial screening test, the reaction of p-phenylenediamine with 2 equiv. of N,N0 -diisopropylcarbodiimide to give the bisguanylation product 4 was chosen. The biguanidine derivative 2,20 (1,4-phenylene)bis(20 ,3-diisopropylguanidine) (4) has been previously obtained by means of titanium or lanthanide complex catalysis [18]. All four new lanthanide-bis(cyclopropylethinylamidinates) 2aec and 3 were used as precatalysts, and the reactions were carried out in concentrated THF solutions at room temperature and at 60  C. The results are summarized in Table 2. It was surprising to find that virtually all of the reactions using the chloro-bridged dimers [{cC3H5eC^CeC(NR)2}2Ln(m-Cl)(THF)]2 (2a: Ln ¼ Ce, R ¼ iPr; 2b: Ln ¼ Ce, R ¼ Cy; 2c: Ln ¼ Nd, R ¼ Cy) proceeded very rapidly and efficiently under the chosen reaction conditions. Reaction times between 15 and 30 min were found to be sufficient to afford nearly quantitative yields of the bis-guanylation product 4. The purity of 4 was confirmed by comparison of its NMR spectra (1H, 13C; see the Supplementary Material) with those reported in the literature [17c,18a]. Although the data are far from being comprehensive, some trends can be noted. Slightly increased isolated yields of 4 (98 to >99% vs. 92 to >99%) were obtained when the catalyst loading was

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10 Table 2 Addition of 1,4-Diaminobenzene to N,N0 -Diisopropylcarbodiimide catalyzed by the Lanthanide-bis(cyclopropylethinylamidinates) 2aec and 3.

Entrya

Cat.

Catalyst loading (mol %)

Temp ( C)

Time (h)

Yieldb of 4 (%)

1 2 3 4 5 6 7 8 9 10 11

2a 2a 2b 2b 2c 2c 2c 2c 3 3 none

0.5 1 0.5 1 0.5 1 0.5 1 0.5 1 0

60 60 60 60 r.t. r.t. 60 60 60 60 60

0.5 0.5 0.5 0.25 0.5 0.5 0.5 0.5 0.5 0.5 0.5

92 98 92 >99 92 93 >99 >99 78 55 0

a b

In THF as solvent. Isolated yield.

increased from 0.5 to 1 mol %, but the differences were marginal. The highest yields (>99%) were achieved with the neodymium precatalyst 2c, although here again the difference to the cerium precatalysts was marginal (92 to >99%). In the case of the Nd precatalyst 2c, an increase of the reaction temperature from r.t. to 60  C lead to an increase of the isolated yield of 4 from 92 to 93% to quantitative conversion (>99%). In marked contrast, use of the holmium precatalyst 3 led to significant lower yields of 4. The initial screening test outlined above had the remarkable result that especially the dimeric chloro-bridged lanthanide-bis(cyclopropylethinylamidinates) 2aec are extremely active precatalysts for the guanylation of aromatic amines under the chosen conditions. This is the more remarkable as they are chloro precursors and not highly reactive amido or alkyl derivatives made thereof. Future work will be directed to the question if the wellknown electron-donating ability of the cyclopropyl group [20] in the cyclopropylethinyl ligands has a reactivity-enhancing effect. In a second set of experiments, the Ln-catalyzed guanylation of several other substituted anilines with both N,N0 -diisopropylcarbodiimide and N,N0 -dicyclohexylcarbodiimide was studied. For these tests the most active complex [{cC3H5eC^CeC(NCy)2}2Nd(m-Cl)(THF)]2 (2c) was used as the precatalyst. The reactions were again carried out in concentrated THF solution at 60  C using a catalyst loading of 0.5 mol %. Table 3 summarizes the results. The resulting known guanidine products were purified by recrystallization and characterized by their 1H and 13 C NMR data (see the Supplementary material). All N,N0 ,N00 trisubstituted guanidines and bis-guanidines listed in Table 3 are known compounds [17,18]. In the case of o- and m-phenylenediamine as substrates (Table 3, entries 1e4), the yields obtained with N,N0 -dicyclohexylcarbodiimides were lower than those obtained with N,N0 -diisopropylcarbodiimide. It is, however, unlikely that with the guanidine units being in p- and m-positions the steric bulk of the substituents on the carbodiimides has a significant influence on the outcome of these reactions. Notable is the outcome of the analogous reactions of o-phenylenediamine with carbodiimides (Table 3, entries 5 and 6). It has been previously reported that the reaction of o-phenylenediamine with N,N0 -diisopropylcarbodiimide in the presence of a titanacarborane monoamide catalyst (10 mol %) directly afforded the cyclic

5

guanidine derivative o-C6H4(NH)2C]NiPr [21]. However, under mild conditions the intermediate mono-guanylation product 8 could be isolated in 59% yield. Thermal treatment of 8 at 140  C for 30 h led to conversion into the cyclic guanidine o-C6H4(NH)2C]NiPr (Scheme 4). This experiment implied that the catalyst might not be involved in the cyclization step [21]. In order to avoid subsequent cyclization in our study, the Ln-catalyzed reaction of o-phenylenediamine with N,N0 -diisopropylcarbodiimide was carried out at room temperature. Quite remarkably, even with a catalyst loading as low as 0.5 mol % the mono-guanylation product 8 could be isolated after only 30 min in nearly quantitative yield (93%). Even at 60  C, N,N0 dicyclohexylcarbodiimide did not react with o-phenylenediamine in the presence of 2c. This is in accordance with a previous report that the Ti-catalyzed formation of the cyclic guanidine oC6H4(NH)2C ¼ NCy requires rather harsh reaction conditions (140  C/120 h) [21]. Reactions of p-chloroaniline with both carbodiimides (Table 3, entries 7 and 8) provided the expected guanylation products 9 and 10 under our standard conditions in quantitative yields (99%), reflecting again the exceptionally high catalytic activity of the neodymium complex 2c. In both cases, quantitative conversion was observed after only 30 min. Both p-chlorophenylsubstituted guanidines could be easily isolated in a highly pure form by recrystallization from acetonitrile. In all cases (4e10), acetonitrile was found to be the solvent of choice for obtaining high-purity guanidine products. In addition to their 1H and 13C NMR data, the molecular structures of the trisubstituted guanidine products 8 and 9 were also authenticated by X-ray diffraction (Table 1). The molecular structures are depicted in Figs. 4 (8) and 5 (9). In both molecules the central structural unit comprises a CN3 core with two distinctly different CeN bonds. Of particular interest is the structure of the o-phenylenediamine-derived compound oC6H4(NH2)[N]C(NHiPr)2] (8). With 1.379(12) and 1.370(12) Å the C1eN2 and C1eN3 bond lengths are consistent with the presence of CeNH single bonds, whereas the C1eN1 bond (1.301(12) Å) clearly has carbonenitrogen double bond character. The CN3 core is nearly planar. This is indicated by the N1eC1eN2, N1eC1eN3, and N2eC1eN3 angles of 119.35(9) , 127.33(9) , and 113.32(8) respectively. These values are in excellent agreement with those reported earlier for similar compounds [7b,17b,19,22] as well as the molecular structure of the p-chlorophenyl derivative p-C6H4Cl[-N] C(NHiPr)2] (9). As mentioned above, compound 8 itself is interesting as an intermediate in the formation of the cyclic guanidine derivative o-C6H4(NH)2C]NiPr according to Scheme 4 [21]. Although the distance H4B$$$$N3 (cf. Fig. 5) is quite long (2.352 Å), it might be interesting to note that the N3eC1eN2 unit is not in a perpendicular arrangement with respect to the phenyl ring plane, but is tilted by 24.8 so that the iPrNH functionality bearing N3 is inclined toward the amino group in ortho-position. In accordance with previous studies we propose the following mechanism for the Ln-catalyzed reaction of aromatic amines with N,N0 -diisopropylcarbodiimide (Scheme 5). In detail, it is safe to assume that the chloro-bridged dimer is first split into monomers upon addition of the aromatic amine to afford adduct A. As in previous cases, the catalytically active species should be a diamido complex B, which in our case could be generated by elimination of the free cyclopropylethinylamidines cC3H5eC^CeC(NHR) (¼NR) (11a,b: R ¼ iPr, Cy). Preliminary positive proof for this assumption came from the independent preparation of c-C3H5eC^CeC(NHCy) (¼NCy) 11b by careful hydrolysis of 1b in acetonitrile according to Scheme 6. Amidine 11b was identified by its 1H and 13C NMR data. A control NMR-tube reaction between 2c and p-chloroaniline in THF-d8 clearly revealed the formation of free

6

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10

Table 3 Addition of primary amines to carbodiimides catalyzed by complex 2c.

Entrya

R

Time (h)

Product (Refs)

Yield %b

1

i

Pr

0.5

4 [17b,18b,18d,18f]

>99

2

Cy

0.5

5 [17b,18b,18d,18f]

79

3

i

Pr

1

6 [17k,18b]

83

4

Cy

1

7 [17k]

76

5

i

Pr

0.5

8 [21]

93c

6

Cy

2

e

Traces

7

i

Pr

0.5

9 [17b,17i,17k]

99

Cy

0.5

10 [17b,17i,17k]

99

8 a b c

Amines

Guanidines

General conditions: Solvent THF, temperature at 60  C and catalyst loading 0.5 mol %. Isolated yield. Reaction carried out at room temperature.

amidine 11b. Thus far, however, we have been unable to isolate an intermediate diamido complex of the type B from the reaction mixture. In the course of a closely related study by Xi et al., a key intermediate with two bridging PhNH amido ligands, [{PhC(NAr)2}2Lu(m-NHPh)]2 (Ar ¼ p-C6H4Me), could be isolated and structurally characterized [19b]. In this case the amido intermediate was formed by elimination of the cyclopentadienyl ligand Cp' from the

precursor Cp0 Lu[PhC(NAr)2]2 upon treatment with aniline. The following steps of the proposed catalytic cycle are well established through a variety of previous studies [17e19,23]. Subsequent insertion of the carbodiimide C]N bond into an LneN bond of the diamido species B could afford a bis(guanidinate)LnCl intermediate C, which upon reaction with the aromatic amine would release the trisubstituted guanidine product (5e10) and regenerate the amido complex 2.

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10

7

Scheme 4. Thermal conversion of 8 into the cyclic guanidine o-C6H4(NH)2C]NiPr [21].

Conclusions In summarizing the work reported here, we succeeded in the straightforward preparation of a series of new lanthanide bis(cyclopropylethinylamidinate) complexes. The lithiumcyclopropylethinylamidinate precursors employed in these preparations are readily available in one step from commercially available starting materials. Dimeric chloro-bridged complexes were obtained for the large lanthanides cerium and neodymium, whereas the smaller holmium afforded the “ate” complex 3. In addition, we could demonstrate that these complexes are efficient catalysts for the guanylation of aromatic amines, with the neodymium precatalyst 2c exhibiting the highest catalytic activity. This is a rare case of lanthanide chloro complexes being active catalysts for guanylation reactions. It eliminates the need for preparing the usually more reactive amido or alkyl derivatives. These findings can also be seen as a first indication that the well-known electrondonating ability of the cyclopropyl group [20] in the cyclopropylethinyl ligands exerts a beneficial electronic effect on the reactivity of rare-earth metal amidinato complexes. The new complexes 2aec and 3 as well as the guanidines 8 and 9 have been structurally characterized by X-ray diffraction. Experimental section General procedures All experiments were carried out in oven-dried or flame-dried glassware under an inert atmosphere of dry argon employing standard Schlenk and glovebox techniques (<1 ppm O2, <1 ppm H2O). Pentane, diethyl ether, and THF were distilled from sodium/ benzophenone under nitrogen atmosphere prior to use, while

Fig. 4. X-ray molecular structure of o-C6H4(NH2)[-N]C(NHiPr)2] (8). Selected bond lengths (Å) and angles ( ): C1eN1 1.301(13), C1eN2 1.379(12), C1eN3 1.370(12), C2eN1 1.414(12), C3eN4 1.399(14), N2eC8 1.463(12), N3eC11 1.473(12); N1eC1eN2 119.35(9), N1eC1eN3 127.33(9), N2eC1eN3 113.32(8), C1eN1eC2 123.84(8), C1eN2eC8 122.83(8), C1eN3eC11 123.75(8), C2eC3eN4 120.79(9).

Fig. 5. Molecular structure of 1,4-C6H4Cl[-N]C(NHiPr)2] (9).

acetonitrile was dried using P4O10. All glassware was oven-dried at 120  C for at least 24 h, assembled while hot, and cooled under vacuum prior to use. Lithium-cyclopropylethinylamidinates 1a and 1b were prepared according to literature methods [9]. N,N0 -Diisopropylcarbodiimide, N,N0 -dicyclohexylcarbodiimide, 4chloroaniline, as well as the phenylenediamines (o, m, and p) were obtained from commercial sources and used as received. 1H NMR (400 MHz) and 13C NMR (100.6 MHz) spectra were recorded in Toluene-d8, C6D6, THF-d8, or CDCl3 solutions on a Bruker DPX 400 spectrometer at 25  C. Chemical shifts were referenced to TMS. Assignment of signals was made from 1He13C HSQC 2D NMR experiments. IR spectra were recorded using KBr pellets on a Perkin Elmer FT-IR spectrometer system 2000 between 4000 cm1 and 400 cm1. Microanalyses of the compounds were performed using a Leco CHNS 923 apparatus. [{ceC3H5eC^CC(NiPr)2}2Ce(m-Cl) (THF)]2 (2a) A solution of anhydrous CeCl3 (1.0 g, 4.1 mmol) in 30 mL of THF was added to a solution of 1a (1.6 g, 8.2 mmol) in 70 mL of THF. The reaction mixture was heated to 65  C for 2 h and stirred at r.t. for 12 h. The solution color changed to yellow. The solvent was removed under vacuum followed by extraction with n-pentane (30 mL) to give a clear, bright yellow solution. The filtrate was concentrated under vacuum to ca. 10 mL. Crystallization at 30  C for three weeks afforded 2a in the form of bright yellow needle-like crystals in 62% yield (1.6 g). Anal. calcd. for C56H92Ce2Cl2N8O2 (1260.52 g mol1): C, 53.31; H, 7.29; N, 8.88. Found: C, 53.29; H, 7.11; N, 8.79. 1H NMR (400 MHz, C6D6, 25  C, TMS) d 11.88 (m, 8H, CH(CH3)2), 3.42 (m, 4H, CH, c-C3H5), 2.67 (m, 8H, CH2, c-C3H5), 1.75 (m, 8H, CH2, c-C3H5), e2.98 (s, br, 48H, CH3) (THF signals not observed). 13C{1H} NMR (100.6 MHz, C6D6, 25  C, TMS) d 171.8 (NCN), 110.2 (C^CeC), 79.1 (HCeC^C) 55.6 (CH(CH3)2), 22.9 (CH(CH3)2), 11.3 (CH2, c-C3H5), 3.0 (CH, c-C3H5) (THF signals not observed). IR (KBr): nmax/cm1 3852w, 3440w, 3282w, 3095w, 2963vs, 2867s, 2608w, 2221vs(C^C), 1613s(C]N), 1465s, 1330 m, 1182 m, 966s, 812 m, 688 m, 527w. [{ceC3H5eC^CC(N-c-C6H11)2}2Ce(m-Cl) (THF)]2 (2b) A solution of anhydrous CeCl3 (1.0 g, 4.1 mmol) in 30 mL of THF was added to a solution of 1b (2.3 g, 8.2 mmol) in 60 mL of THF following the procedure given for 2a. Crystallization at 30  C for

8

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10

Scheme 5. Proposed mechanism of the Ln-Catalyzed Guanylation of Aromatic Amines.

one month afforded 2b in the form of bright yellow, needle-like crystals in 55% yield (1.7 g). Anal. calcd. for C80H124Ce2Cl2N8O2 (1581.01 g mol1): C, 60.72; H, 7.84; N, 7.08. Found: C, 60.86; H, 7.72; N, 7.53. 1H NMR (400 MHz, THF-d8, 25  C, TMS) d 12.78 (m, 8H, CH, Cy), 4.02 (m, 4H, CH, c-C3H5), 3.58 (m, 8H, THF), 3.22 (m, 8H, CH2, c-C3H5), 2.53 (m, br, 8H, CH2, c-C3H5), 1.86e2.21 (m, 24H, CH2, Cy), 1.71e1.80 (m, 8H, THF), 0.96e1.45 (m, 24H, CH2, Cy), 0.86e0.97 (m, 8H, CH2, Cy), e0.54 (m, 8H, CH2, Cy), 3.02 (m, 16H, CH2, Cy). 13C

Scheme 6. Formation of free amidine 11b by controlled hydrolysis of 1b.

{1H} NMR (100.6 MHz, THF-d8, 25  C, TMS) d 109.3 (NCN), 82.3 (C^CeC), 71.5 (HCeC^C), 68.1 (THF) 55.8 (CH, Cy), 37.2 (CH2, Cy), 26.9 (CH2, Cy), 26.5 (THF), 26.3 (CH2, Cy), 11.5 (CH2, c-C3H5), 3.8 (CH, c-C3H5). IR (KBr): nmax/cm1 3677s, 3438 m, 2928vs, 2852vs, 2226s(C^C), 1607vs(C]N), 1482 m, 1390w, 1365w, 1254s, 1157w, 956s, 889s, 685 m, 599w, 464w. [{ceC3H5eC^CC(N-c-C6H11)2}2Nd(m-Cl) (THF)]2 (2c) A 250 mL Schlenk flask was charged with anhydrous NdCl3 (1.0 g, 4 mmol) and 1b (2.2 g, 8 mmol). The reaction mixture in 100 mL THF was heated to 65  C for 3 h. The solution color changed to pale green. The solvent was removed under vacuum to dryness followed by extraction with n-pentane (3  20 mL) to give a clear green solution. The filtrate was concentrated under vacuum to ca. 15 mL. Crystallization at 30  C for two weeks afforded 2c in the form of deep green, needle-like crystals in 85% yield (2.9 g). Anal. calcd. for C80H124Cl2Nd2N8O2 (1589.25 g mol1): C, 60.40; H, 7.80; N, 7.04. Found: C, 60.22; H, 7.30; N, 7.34. 1H NMR (400 MHz, C6D6, 25  C, TMS) d 21.84 (s, 8H, CH, Cy), 4.07 (m, 4H, CH, c-C3H5), 3.03 (m,

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10

8H, CH2, c-C3H5), 2.04 (m, 8H, CH2, c-C3H5), 0.18e0.80 (m, 40H, CH2, Cy), e 0.07 (m, 8H, CH2, Cy), 1.30 (m, 8H, CH2, Cy), 2.65 (m, 8H, CH2, Cy), 8.70 (m, 16H, CH2, Cy) (THF signals not observed). 13C {1H} NMR (100.6 MHz, C6D6, 25  C, TMS) d 119.8 (NCN), 108.0 (C^CeC), 74.1 (CH, Cy), 61.8 (HCeC^C), 36.1 (CH2, Cy), 26.3 (CH2, Cy), 22.6 (CH2, Cy), 12.2 (CH2, c-C3H5), 2.6 (CH, c-C3H5) (THF signals not observed). IR (KBr): nmax/cm1 3438w, 3094w, 3012w, 2925vs, 2851vs, 2664w, 2221vs(C^C), 1608 m(C]N), 1473 m, 1398 m, 1362s, 1343 m, 1307 m, 1174s, 1120s, 973vs, 888 m, 676s, 589s. [ceC3H5eC^CC(N-c-C6H11)2]2Ho(m-Cl)2Li(THF) (Et2O)] (3) Anhydrous HoCl3 (0.5 g, 1.8 mmol) and 1b (1 g, 3.7 mmol) in 100 mL THF were charged into a 250 mL Schlenk flask. The reaction mixture was heated to 65  C for 3 h and stirred at r.t. for 12 h. The solvent was removed under vacuum followed by extraction with diethyl ether (2  20 mL) to give a clear, bright yellow solution. The solvent was removed affording 3 in 83% yield (1.3 g) as bright yellow solid. Anal. calcd. for C44H72Cl2HoLiN4O2 (931.86 g mol1): C, 56.71; H, 7.79; N,6.01. Found: C, 56.79; H, 7.77; N, 6.11. IR (KBr): nmax/cm1 3438w, 3279w, 3220w, 3010w, 2929vs, 2853s, 2227vs(C^C), 1629 m(C]N), 1593 m, 1449vs, 1365 m, 1254w, 974 m, 690w. General procedure for the catalytic reaction of 1,4-diaminobenzene with N,N0 -diisopropylcarbodiimide by 2a, 2b, 2c, or 3 A 100 mL Schlenk flask was charged with 1,4-diaminobenzene (0.7 g, 6.4 mmol) and N,N0 -diisopropylcarbodiimide (2.0 mL, 12.8 mmol) in 20 mL of THF. To the mixture was added the complex (2a, 2b, 2c, or 3) (0.5% mmol), dissolved in 5 mL of THF. The resulting mixture was stirred at 60  C or at room temperature for a fixed time, as shown in Table 2. The solvent was removed under vacuum to dryness. The product was purified by crystallization from a minimum amount of dry acetonitrile in air to give 4 in yields as shown in Table 2 (for NMR data see the Supplementary material). General procedure for the direct synthesis of guanidines from the reaction of primary amines with carbodiimides catalyzed by 2c A 50 mL Schlenk flask under dried argon was charged with primary amines (1.0 equiv.) and carbodiimides (2.0 equiv.) and stirred in 20 mL of THF. To the mixture was added the complex 2c (0.005 equiv.) dissolved in 5 mL THF. The resulting mixture was stirred at 60  C for a fixed time, as shown in Table 3. The solvent was removed under vacuum to dryness. The product was purified by recrystallization from dry acetonitrile in air affording the guanidines in yield as shown in Table 3 (for NMR data see the Supplementary material). General procedure for the direct synthesis of guanidines from the reaction of 4-chloroaniline with carbodiimides catalyzed by 2c Under argon, a 50 mL Schlenk flask was charged with 4-chloroaniline (1.0 equiv.) and carbodiimides (1.0 equiv.) in 10 mL of THF. To the reaction mixture was added the complex 2c (0.5% mmol), dissolved in 5 mL THF. The resulting mixture was stirred at 60  C for a fixed time, as shown in Table 3. The solvent was removed under vacuum and the product was purified by recrystallization from acetonitrile in air affording the guanidines in very high yields as shown in Table 3 (for NMR data see the Supplementary material). Formation of free amidine 11b by controlled hydrolysis of 1b A 0.5 g sample of 1b was dissolved in acetonitrile and hydrolyzed by addition of one drop (ca. 30 mg) of water. Cooling to 20  C afforded colorless crystals of 11b (ca. 100 mg) which were isolated by filtration, dried under vacuum and characterized by NMR spectroscopy. 1H NMR (400 MHz, C6D6, 25  C, TMS) d 3.83e3.96 (s, br, 2H, CH, Cy), 1.03e1.76 (m, 20H, Cy), 0.96e1.03 (m,

9

1H c-C3H5), 0.55e0.60 (m, 2H, c-C3H5), 0.34e0.40 (m, 2H, c-C3H5) (NH not observed); 13C NMR (100.6 MHz, C6D6, 25  C, TMS) d 140.7 (NCN), 94.9 (HeCeC^C), 68.2(C^CeC), 49.1 (CH, Cy), 34.8 (CH2, Cy), 26.4 (CH2, Cy), 25.1 (CH2, Cy), 8.9 (CH2, c-C3H5), 0.35 (CH, cC3H5). X-ray crystallographic studies The intensity data of 2a, 2c, 3 and 9 were collected on a Stoe IPDS 2T diffractometer with MoKa radiation. The data were collected with the Stoe XAREA [24] program using u-scans. The space groups were determined with the XRED32 [24] program. The intensity data of 2b and 8 were registered on an Oxford Diffraction Nova A diffractometer using mirror-focussed CuKa radiation. Absorption corrections were applied using the multi-scan method. The structures were solved by direct methods (SHELXS-97) [25a] and refined by full matrix least-squares methods on F2 using SHELXL-97 [25b]. Data collection parameters are given in Table 1. Acknowledgments €t MagFinancial support by the Otto-von-Guericke-Universita deburg is gratefully acknowledged. Farid M. Sroor is grateful to the ministry of Higher Educational scientific Research (MHESR), Egypt, and the Germany Academic Exchange Service (DAAD), Germany, for a Ph.D. scholarship within the German Egyptian Research LongTerm Scholarship (GERLS) program. Appendix A. Supplementary data CCDC-1001399 (2a), 1001400 (2b), 1001401 (2c), 1001403 (3), 1001402 (8), and 1001404 (9) contain the supplementary crystallographic data for this Paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/products/csd/request. Appendix B. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2015.01.034. References [1] Review articles: (a) K. Dehnicke, Chem.-Ztg. 114 (1990) 295e304; (b) J. Barker, M. Kilner, Coord. Chem. Rev. 133 (1994) 219e300; (c) F.T. Edelmann, Coord. Chem. Rev. 137 (1994) 403e481; (d) F.T. Edelmann, Top. Curr. Chem. 179 (1996) 113e148; (e) H. Nagashima, H. Kondo, T. Hayashida, Y. Yamaguchi, M. Gondo, S. Masuda, K. Miyazaki, K. Matsubara, K. Kirchner, Coord. Chem. Rev. 245 (2003) 177e190; (f) P.W. Roesky, Z. Anorg. Allg. Chem. 629 (2003) 1881e1894; (g) F.T. Edelmann, Adv. Organomet. Chem. 57 (2008) 183e352; (h) M.P. Coles, Chem. Commun. (2009) 3659e3676; (i) C. Jones, Coord. Chem. Rev. 254 (2010) 1273e1289; (j) A.A. Trifonov, Coord. Chem. Rev. 254 (2010) 1327e1347; (k) A.A. Mohamed, H.E. Abdou Jr., J.P. Fackler, Coord. Chem. Rev. 254 (2010) 1253e1259; (l) S. Collins, Coord. Chem. Rev. 255 (2011) 118e138; (m) F.T. Edelmann, Adv. Organomet. Chem. 61 (2013) 55e374. [2] (a) F.T. Edelmann, Chem. Soc. Rev. 38 (2009) 2253e2268; (b) F.T. Edelmann, Chem. Soc. Rev. 41 (2012) 7657e7672. [3] (a) H. Fujita, R. Endo, A. Aoyama, T. Ichii, Bull. Chem. Soc. Jpn. 45 (1972) 1846e1852; (b) G. Himbert, M. Feustel, M. Jung, Liebigs Ann. Chem. (1981) 1907e1927; (c) G. Himbert, W. Schwickerath, Liebigs Ann. Chem. (1984) 85e97; (d) G.F. Schmidt, G. Süss-Fink, J. Organomet. Chem. 356 (1988) 207e211; (e) T.-G. Ong, J.S. O'Brien, I. Korobkov, D.S. Richeson, Organometallics 25 (2006) 4728e4730; (f) X. Xu, J. Gao, D. Cheng, J. Li, G. Qiang, H. Guo, Adv. Synth. Catal. 350 (2008) 61e64; €se, C.G. Hrib, F.T. Edelmann, J. Organomet. Chem. 695 (2010) (g) P. Dro 1953e1956;

10

[4]

[5] [6]

[7]

[8] [9] [10]

[11]

[12]

[13] [14]

[15] [16]

[17]

F.M. Sroor et al. / Journal of Organometallic Chemistry 785 (2015) 1e10 €rtner, W. Kantlehner, G. Maas, Synthesis (2011) 265e272; (h) W. Weinga €rtner, G. Maas, Eur. J. Org. Chem. (2012) 6372e6382. (i) W. Weinga (a) H. Fujita, R. Endo, K. Murayama, T. Ichii, Bull. Chem. Soc. Jpn. 45 (1972) 1581; (b) W. Ried, M. Wegwitz, Liebigs Ann. Chem. (1975) 89e94; (c) W. Ried, R. Schweitzer, Chem. Ber. 109 (1976) 1643e1649; (d) W. Ried, H. Winkler, Chem. Ber. 112 (1979) 384e388. T. Ichii, H. Fujita, R. Endo, A. Aoyama, H. Takagi, K. Kamoshida, Jpn. Pat. Appl. JP 71 (1972) 13775, 19710313. (a) P. Sienkiewich, K. Bielawski, A. Bielawska, J. Palka, Environ. Toxicol. Pharmacol. 20 (2005) 118e124; (b) T.M. Sielecki, J. Liu, S.A. Mousa, A.L. Racanelli, E.A. Hausner, R.R. Wexler, R.E. Olson, Bioorg. Med. Chem. Lett. 11 (2001) 2201e2204; (c) C.E. Stephens, E. Tanious, S. Kim, D.W. Wilson, W.A. Schell, J.R. Perfect, S.G. Franzblau, D.W. Boykin, J. Med. Chem. 44 (2001) 1741e1748; (d) C.N. Rowley, G.A. DiLabio, S.T. Barry, Inorg. Chem. 44 (2005) 1983e1991. (a) W.-X. Zhang, M. Nishiura, Z. Hou, J. Am. Chem. Soc. 127 (2005) 16788e16789; (b) S. Zhou, S. Wang, G. Yang, Q. Li, L. Zhang, Z. Yao, Z. Zhou, H.-B. Song, Organometallics 26 (2007) 3755e3761; (c) W.-X. Zhang, Z. Hou, Org. Biomol. Chem. 6 (2008) 1720e1730; (d) Z. Du, W. Li, X. Zhu, F. Xu, Q. Shen, J. Org. Chem. 73 (2008) 8966e8972; (e) C.N. Rowley, T.-G. Ong, J. Priem, D.S. Richeson, T.K. Woo, Inorg. Chem. 47 (2008) 12024e12031; (f) Y. Wu, S. Wang, L. Zhang, G. Yang, X. Zhu, Z. Zhou, H. Zhu, S. Wu, Eur. J. Org. Chem. (2010) 326e332; (g) L. Xu, Y.-C. Wang, W.-X. Zhang, Z. Xi, Dalton Trans. 42 (2013) 16466e16469. W.W. Seidel, W. Dachtler, T. Pape, Z. Anorg. Allg. Chem. 638 (2012) 116e121. F.M.A. Sroor, C.G. Hrib, L. Hilfert, F.T. Edelmann, Z. Anorg. Allg. Chem. 639 (2013) 2390e2394. (a) R. Duchateau, C.T. van Wee, A. Meetsma, J.H. Teuben, J. Am. Chem. Soc. 115 (1993) 4931e4932; (b) F.T. Edelmann, J. Richter, Eur. J. Solid State Inorg. Chem. 33 (1996) 157e163. (a) Z.P. Lu, G.P.A. Yap, D.S. Richeson, Organometallics 20 (2001) 706e712; (b) Y. Luo, Y. Yao, Q. Shen, L. Weng, Eur. J. Inorg. Chem. (2003) 318e323; (c) A.A. Trifonov, E.A. Fedorova, G.K. Fukin, M.N. Bochkarev, Eur. J. Inorg. Chem. (2004) 4396e4401; (d) Y.-M. Yao, Y.-J. Luo, Q. Shen, K.-B. Yu, Jiegou Huaxue 23 (2004) 391; (e) X. Pang, H. Sun, Y. Zhang, Q. Shen, H. Zhang, Eur. J. Inorg. Chem. (2005) 1487e1491. (a) Y. Yao, Y. Luo, J. Chen, Z. Zhang, Y. Zhang, Q. Shen, J. Organomet. Chem. 679 (2003) 229e237; (b) A.A. Trifonov, D.M. Lyubov, E.A. Fedorova, G.K. Fukin, H. Schumann, S. Mühle, M. Hummert, M.N. Bochkarev, Eur. J. Inorg. Chem. (2006) 747e756; (c) P. Benndorf, J. Jenter, L. Zielke, P.W. Roesky, Chem. Commun. 47 (2011) 2574e2756. G.K. Fukin, I.A. Guzei, E.V. Baranov, J. Coord. Chem. 60 (2007) 937e944. , D.M. Lyubov, A.M. Bubnov, G.K. Fukin, F.M. Dolgushin, M.Y. Antipin, O. Pelce M. Schappacher, S.M. Guillaume, A.A. Trifonov, Eur. J. Inorg. Chem. (2008) 2090e2098. (a) F.T. Edelmann, Struct. Bond 137 (2010) 109e163; (b) R.Z. Kempe, Anorg. Allg. Chem. 636 (2010) 2135e2147. Review articles: (a) H. Shen, Z. Xie, J. Oganomet. Chem. 694 (2009) 1652e1658; (b) T. Suzuki, W.-X. Zhang, M. Nishiura, Z. Hou, J. Synth. Org. Chem. Jpn. 67 (2009) 451e464. (a) W.-X. Zhang, M. Nishiura, Z. Hou, Synlett (2006) 1213e1216; (b) W.-X. Zhang, M. Nishiura, Z. Hou, Chem. Eur. J. 13 (2007) 4037e4051; (c) Q. Li, S. Wang, S. Zhou, G. Yang, X. Zhu, Y. Liu, J. Org. Chem. 72 (2007) 6763e6767;

[18]

[19]

[20]

[21] [22] [23]

[24] [25]

(d) X. Zhu, Z. Du, F. Xu, Q. Shen, J. Org. Chem. 74 (2009) 6347e6349; (e) Y. Wu, S. Wang, L. Zhang, G. Yang, X. Zhu, C. Liu, C. Yin, J. Rong, Inorg. Chim. Acta 362 (2009) 2814e2819; (f) R. Jiao, M. Xue, X. Shen, Y. Zhang, Y. Yao, Q. Shen, Eur. J. Inorg. Chem. 17 (2010) 2523e2529; (g) C. Qian, X. Zhang, Y. Zhang, Q. Shen, J. Organomet. Chem. 695 (2010) 747e752; (h) C. Liu, S. Zhou, S. Wang, L. Zhang, G. Yang, Dalton Trans. 39 (2010) 8994e8999; (i) Y. Cao, Z. Du, W. Li, J. Li, Y. Zhang, F. Xu, Q. Shen, Inorg. Chem. 50 (2011) 3729e3737; (j) F. Han, Q. Teng, Y. Zhang, Y. Wang, Q. Shen, Inorg. Chem. 50 (2011) 2634e2643; (k) X. Zhang, C. Wang, C. Qian, F. Han, F. Xu, Q. Shen, Tetrahedron 67 (2011) 8790e8799; (l) L. Xu, Z. Wang, W.-X. Zhang, Z. Xi, Inorg. Chem. 51 (2012) 11941e11948; (m) J. Tu, W. Li, M. Xue, Y. Zhang, Q. Shen, Dalton Trans. 42 (2013) 5890e5891. (a) T.G. Ong, G.P.A. Yap, D.S. Richeson, J. Am. Chem. Soc. 125 (2003) 8100e8101; (b) S. Hao, H.-S. Chan, Z. Xie, Organometallics 25 (2006) 5515e5517; (c) C.N. Rowley, T.-G. Ong, J. Priem, T.K. Woo, D.S. Richeson, Inorg. Chem. 47 (2008) 9660e9668; (d) D. Li, J. Guang, W.-X. Zhang, Y. Wang, Z. Xi, Org. Biomol. Chem. 8 (2010) 1816e1820; (e) D. Elorriaga, F. Carrillo-Hermosa, A. Antinolo, I. Lopez-Solera, B. Menot, R. Fernandez-Galan, E. Villasenor, A. Otero, Organometallics 31 (2012) 1840e1848; (f) L. Zhi, M. Xue, H. Yao, H. Sun, Y. Zhang, Q. Shen, J. Organomet. Chem. 713 (2012) 27e34. (a) C. Pi, Z. Zhang, Z. Pang, J. Zhang, J. Luo, Z. Chen, L. Weng, X. Zhou, Organometallics 26 (2007) 1934e1946; (b) P.H. Wei, L. Xu, L.-C. Song, W.-X. Zhang, Z. Xi, Organometallics 33 (2014) 2784e2789. Selected references: (a) P. Fischer, W. Kurtz, F. Effenberger, Chem. Ber. 106 (1973) 549e559; (b) C.F. Wilcox, L.M. Loew, R. Hoffman, J. Am. Chem. Soc. 95 (1973) 8192e8193; (c) A. de Meijere, Angew. Chem. Int. Ed. 18 (1979) 809e826; (d) H.C. Brown, M. Periasamy, P.T. Perumal, J. Org. Chem. 49 (1984) 2754e2757; (e) D.D. Roberts, J. Org. Chem. 56 (1991) 5661e5665; (f) R.R. Sauers, Tetrahedron 54 (1998) 337e348; (g) A. de Meijere, S.I. Kozhushkov, Mendeleev Commun. 20 (2010) 301e311. H. Shen, Y. Wang, Z. Xie, Org. Lett. 13 (2011) 4562e4565. €hn, M.S. Hill, J.R. Lachs, A.G.M. Barrett, M.R. Crimmin, G. Kociok-Ko M.F. Mahon, P.A. Procopiou, Eur. J. Inorg. Chem. (2008) 4173e4179. (a) W.-X. Zhang, M. Nishiura, Z. Hou, Chem. Commun. (2006) 3812e3814; (b) W.-X. Zhang, M. Nishiura, T. Mashiko, Z. Hou, Chem. Eur. J. 14 (2008) 2167e2179; (c) M.R. Crimmin, A.G.M. Barrett, M.S. Hill, P.B. Hitchcock, P.A. Procopiu, Organometallics 27 (2008) 497e499; (d) F. Zhao, Y. Wang, W.-X. Zhang, Z. Xi, Org. Biomol. Chem. 10 (2012) 6266e6270; ~ olo, F. Carrillo-Hermosilla, A. Otero, Chem. Soc. (e) C. Alonso-Moreno, A. Antin Rev. 43 (2014) 3406e3425. Stoe, XAREA Program for Xray Crystal Data Collection, (XRED32 Included in XAREA), Stoe, 2002. (a) G.M. Sheldrick, SHELXL-97 Program for Crystal Structure Refinement, € ttingen, Germany, 1997; Universit€ at Go (b) G.M. Sheldrick, SHELXS-97 Program for Crystal Structure Solution, Uni€t Go €ttingen, Germany, 1997. versita