Hexa- and Octanuclear Heterometallic Clusters with Copperâ, Silver

Article pubs.acs.org/Organometallics

Hexa- and Octanuclear Heterometallic Clusters with Copper−, Silver−, or Gold−Molybdenum Bonds and d10−d10 Interactions Pierre Croizat,† Sabrina Sculfort,† Richard Welter,‡ and Pierre Braunstein*,† †

Laboratoire de Chimie de Coordination and ‡Laboratoire DECOMET, CNRS, Chimie UMR 7177, Université de Strasbourg, 4 rue Blaise Pascal, Strasbourg CEDEX F-67081, France S Supporting Information *

ABSTRACT: A comparative study of the reactivity of the carbonylmetalates {m}− = [MoCp(CO)3]− (Cp = η5-C5H5) and [Mo(η5-C5H4NMe2)(CO)3]− toward d10 complexes of the group 11 metals, [Cu(NCMe)4](BF4), AgBF4 and [N(n-Bu)4][AuBr2], has allowed the characterization of new heterometallic hexa- and octanuclear clusters with the same general formula [M{m}]n (M = Cu, Ag, Au). In these cyclic oligomers, the value of n depends of the coinage metal. Thus, the hexanuclear cluster [Cu3{Mo(η5-C5H4NMe2)(CO)3}3] (17) has a planar metal core, formed by a copper triangle with edge-bridging molybdenum atoms. The octanuclear “star shape” clusters [Ag4{Mo(η5-C5H4NMe2)(CO)3}4] (19) and [Au4{Mo(η5-C5H4NMe2)(CO)3}4] (21) contain a square silver or gold core, respectively, edge-bridged by molybdenum atoms. In these three clusters, the 2-D raft-type structure of their metal core, which is ν2-triangular for Cu3Mo3 and of a square-in-a-square-type for the octanuclear Ag4Mo4 and Au4Mo4 clusters, allows for d10−d10 metallophilic interactions. The latter have been clearly evidenced by relatively short separations between the d10 metal centers: average Cu···Cu, Ag···Ag, and Au···Au distances of 2.617(1), 2.869(1) and 2.792(1) Å, respectively. These new carbonyl clusters are closely related to their Cp analogs previously reported and constitute a unique set of heterometallic clusters with such geometries involving group 6 and group 11 transition metals.

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metal (2.884 Å).7 An alternative bonding description of this triangular PtAu2 cluster involving a formally d10, electron-rich Pt(0) center interacting with two d10 [Au(PPh3)]+ moieties would highlight the aurophilic interactions between the latter. In rarer cases, heterometallic complexes or clusters containing bare Cu(I), Ag(I) or Au(I) centers have been obtained, i.e. in which these atoms are not bound to any ligand and are therefore only involved in homo- and/or heterometallic metal−metal bonding. A class of heterometallic complexes containing bare d10 ions M from the group 11 metals is represented by trinuclear linear or bent chain complexes of the type M′−M−M′ where M′ is a transition or a post-transition metal.8 A further category contains complexes in which the group 11 d10 ion bridges one or two metal−metal bonds, forming triangular or bow-tie clusters, respectively, caps one or two triangular metal faces, or serves as connector between metal polyhedra to form discrete or oligomeric species (Scheme 1).2b,d,9 In heterometallic complexes containing a single d10 ion, additional d10−d10 close-shell interactions can obviously only be of the intermolecular type and their occurrence depends significantly from parameters such as steric hindrance and packing forces, which are generally difficult to estimate and predict. This renders an evaluation of the consequences of d10−d10 interaction(s) on structures and properties more

INTRODUCTION Following the first examples of heterometallic complexes containing a metal−metal bond between a group 11 d10 ion (Cu(I), Ag(I), or Au(I)) and another transition metal, published by Coffey, Lewis and Nyholm in 1964,1 a considerable interest has developed for heterometallic compounds containing these closeshell metal ions, and many of these complexes feature, in addition to the heterometallic bonds, attractive interactions between the d10 metal centers.2 The latter are coined metallophilic interactions, to broaden the early concept of aurophilicity,3 and they have generated increasing interest, from both experimental and theoretical points of view, because of their relevance to a number of research areas.2,4 The most studied d10−d10 interactions are those between gold atoms and the energies involved, between ca. 20−50 kJ/mol, are comparable with that of hydrogen bonding. Aurophilic interactions are considered of dispersion/correlation origin, enhanced by relativistic effects, and the corresponding Au···Au distances should typically be less than twice the van der Waals radius of Au(I) (3.32 Å).3a,b,4s,5 In most cases, the d10 centers are coordinated by ligands and the cationic complex [Au(PPh3)]+ is certainly one of the most popular synthons. This complex was used in e.g. the synthesis of the first platinum gold-cluster, [Pt{Au(PPh3)}2Cl(PEt3)2]+, in which a formally d8 Pt(II) center and a neutral digold unit, isolobal to H2,6 form a two-electron, three-center bonding interaction resulting in a Au−Au distance of 2.737(3) Å, significantly shorter than in gold © XXXX American Chemical Society

Received: September 22, 2016

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DOI: 10.1021/acs.organomet.6b00745 Organometallics XXXX, XXX, XXX−XXX

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[Mo(η5-C5H4NMe2)(CO)3]− (5)14 toward the d10 complexes [Cu(NCMe)4]BF4, AgBF4 and [N(n-Bu)4][AuBr2], are based on previous results which are briefly recalled below. Indeed, a series of isolobal6 or isoelectronic tricarbonylmetalates which differ by the nature of the metal or of the π-bonded ring, the phenylcyclopentadienyl- (6),15 phenylborole- (7),16,17 boratanaphthalene- (8),18 and boratabenzene- (9)14,15 metalates led to a remarkable bonding diversity in a family of planar, centrosymmetric Pd2m2 heterotetranuclear clusters, 10−15, respectively.

Scheme 1. Heterometallic Structures with Naked d10 Ions of the Group 11 Metals M

difficult, and this is one of the reasons why we are particularly interested in intramolecular d10−d10 interactions, which should be less sensitive to such external parameters. Among the metalloligands to be reacted with the group 11 metals to form the desired heterometallic clusters, the group 6 tricarbonylmetalate [MoCp(CO)3]− (1; Cp = η5-C5H5), and its Cr and W analogs, have been successfully used for the synthesis of various metal−metal bonded heterometallic complexes, ranging from heterodinuclear complexes, trinuclear metal chains to clusters.2b−d,10 However, relatively few heterometallic complexes and clusters contain one or more bare d10 metal centers bonded to a M1Cp(CO)3 moiety (M1 = Cr, Mo, and W). These include the anionic trinuclear, linear chain complexes (NMe4)[Cu{M1Cp(CO)3}2] (M1−Cu−M1) (2a−c),11 (NMe4)[Ag{M1Cp(CO)3}2] (M1−Ag−M1) (3a−c)11 and (NEt4)[Au{M1Cp(CO)3}2] (M1−Au−M1) (4a−c),12 and the oligomeric species [{AgMoCp(CO)3}n].11 The structures of salts containing 2b, 3b, 4a, and 4b have been established by X-ray diffraction.8,12b

Whereas in 10,15 11,15 and 13,18 the π-bonded 5-electron donor ring interacts solely with the molybdenum atom, it bridges a Mo−Pd bond in 14 (via the adjacent π-system)15 and in 15 (via the B−N(iPr)2 moiety).15 In the unusual, 54 electron cluster [Re2Pd2(C4H4BPh)2(CO)6] (12), the borole ligand is coordinated to Re in the classical η5 mode but in addition, interacts with the adjacent palladium by forming a 2e−3c B−Cipso−Pd bond.17

In a preliminary report, we described a series of homologous oligomeric clusters of the type [M{m}]n containing the same bridging metalloligand, {m} = MoCp(CO)3.13 The structures of these mixed-metal clusters were characterized by a triangular Cu3 core inscribed within a Mo3 triangle, thus forming a Cu3Mo3 frequency 2 (ν2) triangular structure, and by octanuclear Ag4Mo4 and Au4Mo4 cores containing a central Ag4 or Au4 square, respectively, inscribed within a Mo4 square (square-in-a-square structure). Theoretical studies showed that minor energy differences existed between conformers of different symmetry that were relevant to the extent of d10−d10 interactions.13 Thus, replacing the Cp ligand with a functionalized Cp may be sufficient, for both steric and electronic reasons, to bring about significant structural changes in the corresponding clusters. Further reasons for our interest in a comparative reactivity study between [MoCp(CO)3]− and the amino-substituted derivative B


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Organometallics An example of bridging behavior of 5 involving the molybdenum and the nitrogen atoms was found with the structurally characterized dinuclear complex [Pt{Mo(η5C5H4NMe2)(CO)3}(NCPh)Cl] (16), in which the amino ring substituent interacts with the adjacent platinum center.14

These results formed the basis for our interest to examine whether relatively minor variations in the metalloligand to be reacted with the d10 ions would affect the outcome of the reactions and the structure of the anticipated mixed-metal clusters. Here we show that the metalate [MoCp(CO)3]− (1) and the related [Mo(η5-C5H4NMe2)(CO)3]− (5), selected for its isoelectronic relationship to 9, react very similarly with the various d10 precursor complexes used. This allows a comparison to be made between clusters of the general formula [M−MoCp(CO)3]n (M = Cu, n = 3; M = Ag or Au, n = 4; Cp = η5-C5H5)13 and the new heterometallic clusters [M-Mo(η5-C5H4NMe2)(CO)3]n (M = Cu, n = 3; M = Ag or Au, n = 4). Although no M−N bond involving the amino substituent was formed, interesting structural features were observed. The planar hexanuclear, triangular Cu3Mo3 clusters and the octanuclear, square-in-a-square Ag4Mo4 and Au4Mo4 clusters structurally characterized in this work display significant d10−d10 metallophilic interactions. Together with their Cp analogs,13 these molecules form a unique set of completely analogous groups 6−11 heterometallic clusters, differing only by the nature of the d10 ions involved. The only other directly related set of transition metal/coinage metal clusters contains the Fe(CO)4 fragment.19−21

Figure 1. View of the crystal structure of 17 in 17·1.5(C7H8). Selected bond lengths (Å) and angles (deg): Cu1−Cu2 2.6548(7), Cu1−Cu3 2.579(1), Cu2−Cu3 2.6159(9), Mo1−Cu1 2.6516(7), Mo1−Cu3 2.6272(8), Mo2−Cu1 2.6279(9), Mo2−Cu2 2.653(1), Mo3−Cu2 2.667(2), Mo3−Cu3 2.6559(8); Cu2−Cu1−Cu3 59.95(3), Cu1−Cu2−Cu3 58.60(3), Cu1−Cu3−Cu2 61.46(3), Mo1−Cu1−Mo2 175.50(2), Mo2−Cu2−Mo3 173.41(2), Mo1− Cu3−Mo3 176.85(2).

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RESULTS Copper−Molybdenum Clusters. To evaluate the reactivity of 5 toward group 11 d10 metal complexes, we first reacted the copper(I) complex [Cu(NCMe)4]BF4 with Li·5·2DME in a 1:1 ratio in toluene. The yellow compound isolated from the fraction soluble in toluene was the neutral Cu3Mo3 cluster [Cu3{Mo(η5-C5H4NMe2)(CO)3}3] (17) which has been characterized by single crystal X-ray diffraction (Figure 1 and Table S1). Each Mo(η5-C5H4NMe2)(CO)3 fragment is bonded to two Cu(I) centers and the cluster core adopts a 2-D raft-type arrangement with the central Cu(I) centers interacting with each other (cuprophilic interactions, see below) to form a metal triangle which is inscribed within a molybdenum triangle. Each copper atom being situated in the midpoint of a Mo−Mo segment, this confers to 17 a triangular metal core of frequency 2 (ν2 triangle, Figure 2), like in the analogous cluster [Cu3{MoCp(CO)3}3] (18).13 Each Cu atom has a linear coordination environment, and the Mo(1)Mo(2)Mo(3) triangle is almost equilateral (Mo(1)− Mo(2) = 5.275(1) Å, Mo(2)−Mo(3) = 5.311(3) Å, Mo(1)− Mo(3) = 5.281(1) Å). The average value of the Cu−Mo bond lengths of 2.647(1) Å is similar to that in the analogous cluster 18 (2.642(1) Å but longer than that in the trinuclear chain complex

Figure 2. Orthogonal views of the Cu3Mo3 metal core of 17.

[Na(dme)22b] (2.5810(4) Å).8 The nitrogen atoms of the substituted rings are in a planar environment (sums of the angles around N(1), N(2), and N(3) are 351.2, 356.5, and 355.9°, respectively), and the N−C bonds involving the C5H4 ring, N(1)−C(8) = 1.370(5), N(2)−C(28) = 1.359(5), and N(3)− C(18) = 1.354(5) Å, are all shorter than the N−Cmethyl bonds (N(1)−C(9) = 1.448(6), N(1)−C(10) = 1.451(7), C


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respectively, and also display ν2-triangular structures. The mean d10−d10 distances in all these copper triangles are very similar: 2.626(3) Å in [Cu3Os3H9(PMe2Ph)9],25 2.602(2) Å in [Cu3Fe3(CO)12]3−,19 2.617(1) Å in 17·1.5(C7H8), and 2.627(8) Å in 18. The main distances and angles in 17·1.5(C7H8) are given in Table S1. A significant difference between the cluster with the aminofunctionalized aromatic ring, 17·1.5(C7H8), and 18 is found in the orientation of the metalloligands with respect to the Cu−Cu edge they bridge (Figure 3). Considering the centroids of the

N(2)−C(30) = 1.435(7), N(2)−C(29) = 1.443(7), N(3)− C(19) = 1.447(5), and N(3)−C(20) = 1.453(5) Å), which is consistent with some double bond character between the nitrogen and the C5H4 ring. The bonding of the carbonyls may be analyzed by using criteria commonly adopted to differentiate between terminal, symmetrical μ2, semibridging μ2, semitriply bridging, or triply bridging bonding modes.22 Convenient parameters include the Mo−C−O angle θ and the structural asymmetry parameter α = (D2 − D1)/D1 (Scheme 2).23 Scheme 2. Bonding of the Carbonyl Ligands (α = (D2 − D1)/D1)a

α ≤ 0.6 corresponds to a semi-bridging carbonyl, α ≤ 0.1 corresponds to a bridging carbonyl, and α ≥ 0.6 corresponds to a terminal carbonyl.23 a

Figure 3. Schematic representation of 17 in 17·1.5(C7H8) (left) and 18 (right).

The bonding parameters of the CO ligands in clusters 17, 19, and 21 are detailed in Tables S2, S4, and S5. Each of the three bridging metalloligands Mo(η 5 C5H4NMe2)(CO)3 has a different orientation, as defined by the angle between the Mo−(C 5H 4) centroid axis and the corresponding Cu−Cu edge. The Mo(1) environment is better described as a four-legged piano stool, with the legs being defined by the carbon atoms of the three carbonyls and Cu(3). The Mo(3)(η5-C5H4NMe2)(CO)3 moiety bridges the Cu(2)−Cu(3) edge in a symmetrical fashion, and the metal− metal bonds Mo(3)−Cu(2) and Mo(3)−Cu(3) are situated inside the cone angle defined by the three carbonyls. The environment around Mo(3) is thus best described as a threelegged piano stool. The fragment Mo(2)(η5-C5H4NMe2)(CO)3 bridges the Cu(1)−Cu(2) edge, and the Cu(1)−Mo(2) bond lies inside the Mo(CO)3 cone angle, whereas the Cu(2)− Mo(2) bond is situated slightly outside of this cone. The environment around Mo(2) could therefore be described as intermediate geometry between a three- and a four-legged piano stool. The bridging mode of the carbonyls is directly related to the orientation of the metalloligand, as defined above, and by the rotation of the Mo(CO)3 tripod about its 3-fold axis. In the centrosymmetric Pd2Mo2 and Pt2Mo2 clusters containing the related metalloligand 1, it was observed that this rotation is easy and that various structural conformers can even cocrystallize.24

aromatic rings (CCp) and the center of gravity of the planar Cu3 triangle (Ctriangle), the differences in orientation of the metalloligand are best reflected by the CCp−Mo-Ctriangle angle β whose values are reported in Table 1. Table 1. Values of the Angle β for 17 in 17·1.5(C7H8) and for 18 β

triclinic 17

triclinic 18

C1Cp−Mo1−Ctriangle C2Cp−Mo2−Ctriangle C3Cp−Mo3−Ctriangle

129.06(3) 157.03(4) 162.01(3)

164.00(2) 159.21(2) 160.09(3)

Silver−Molybdenum Clusters. Under conditions similar to those used for the synthesis of 17, the reaction of Li·5·2DME with AgBF4 yielded the pale yellow cluster [Ag4{Mo(η5C5H4NMe2)(CO)3}4] (19), which was characterized by singlecrystal X-ray diffraction (Figure 4 and Table S3). Cluster 19 possesses a C2 symmetry axis passing through the atoms Ag(1) and Ag(3). The central unit of the Ag4Mo4 metal core forms an almost regular silver square and every Ag−Ag edge is bridged by a Mo(η5-C5H4NMe2)(CO)3 moiety in such a way that the inner Ag4 square is inscribed within a distorted Mo4 square, the eight metals thus forming a square-in-a-square structure, as also observed in [Ag4{MoCp(CO)3}4] (20).13 The molybdenum atoms are not coplanar with the inner silver square, since a slight tetrahedral distortion around the Ag centers leads two opposite molybdenum atoms to be located on the same side of the Ag4 plane (Figure 5). The Mo−Ag−Mo angles reveal an inward bending of the Mo− Ag−Mo chains, which is consistent with attractive argentophilic (see below).2q,3c The mean value of the Ag−Mo bond lengths of 2.847(1) Å is similar to that in analogous cluster 20 (2.849(1) Å) but longer than that in the chain complex [Na(dme)]3b (2.787(1) Å).8 The mean d10−d10 distance in the silver square of 19 (2.869(1) Å) is shorter than those in [Ag4{Co(CO)4}4] (3.021(2) Å)26 and [Ag4{Fe(CO)4}4]4− (3.149(1) Å)21 but similar to that in cluster 20 (2.870(1) Å).13

The core structures of 17 and 18 are related to those of the clusters [Cu3Os3H9(PMe2Ph)9]25 and [Cu3Fe3(CO)12]3−,19 which contain bridging OsH3(PMe2Ph)3 and Fe(CO)4 moieties, D


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Figure 5. Representations of the Ag4Mo4 metal core of 19.

Figure 6. Schematic representation of 19 (left) and 20 (right).

Table 2. Values of the Angle β for 19 and 20a β

monoclinic 19

orthorhombic 20

C1Cp−Mo1−Csquare C2Cp−Mo2−Csquare

126.08(2) 138.00(2)

132.66(5) 141.41(5)

C1Cp, C2Cp, C1′Cp, and C2′Cp are the centroids of the Cp-rings (CCp) and Csquare is the centroid of the Ag4 square.

a

well-reflected by the values of the CCp−Mo−Csquare angle β reported in Table 2. Gold−Molybdenum Clusters. The reaction of Li·5·2DME with the gold(I) complex [N(n-Bu)4][AuBr2] in toluene at −40 °C in a 1:1 ratio led instantaneously to the formation of temperature-sensitive, red complex [Au4{Mo(η5-C5H4NMe2)(CO)3}4] (21), which was characterized by single-crystal X-ray diffraction (Figure 7). The octanuclear metal core of cluster 21 has a square-in-asquare-type structure, isotypic to that of 19, with gold(I) replacing silver(I) in the central square. A C2 symmetry axis passes through the atoms Au(1) and Au(3). Views of the crystal structure of 21 are shown in Figures 7 and 8, and the main distances and angles are given in Table S3. The values of the Mo−Au−Mo angles indicate that the gold centers and their adjacent molybdenum atoms are not collinear. The inward bending of the Mo−Au−Mo units is again consistent with attractive aurophilic interactions within the central square (see below). The mean value of the Au−Mo bond lengths of 2.811(1) Å is similar to that in the analogous cluster 22 (2.819(1) Å) but longer than that in the chain complex [NEt4]4b (2.722(1) Å).8 The structures of clusters 21 and [Au4{MoCp(CO)3}4] (22)13 are comparable to that of the tetraanion in [NMe3CH2Ph]4[Au4{Fe(CO)4}4] which also contains an inner gold square, edgebridged by Fe(CO)4 groups.21 The mean d10−d10 distance in the

Figure 4. View of the crystal structure of 19. Selected bond lengths (Å) and angles (deg): Ag1−Ag2 2.8287(7), Ag2−Ag3 2.9088(7), Mo1−Ag1 2.8805(6), Mo1−Ag2 2.8539(8), Mo2−Ag2 2.7994(8), Mo2−Ag3 2.8555(5); Ag2−Ag1−Ag2′ 94.89(3), Ag1−Ag2−Ag3 86.80(2), Ag2− Ag3−Ag2′ 91.51(3), Mo1′−Ag1−Mo1 149.81(4), Mo1−Ag2−Mo2 157.55(3).

The core structures of clusters 19 and 20 are comparable to those of the clusters [Ag4{Co(CO)4}4]26 and [NMe3(CH2Ph)]4 [Ag4{Fe(CO)4}4],20 in which the Ag−Ag edges are bridged by Co(CO)4 or Fe(CO)4 groups, respectively, and which also have a square-in-a-square-type structure. Main distances and angles in 19 are given in Table S3. The bonding of the carbonyl ligands is analyzed in Table S4. As for 17, the planar environment of the N atoms and the values of the N−C bonds are indicative of some double bond character between the nitrogen and the C5H4 ring. A significant difference between clusters 19 and 20 is found in the orientation of the aromatic rings with respect to the Ag−Ag edge bridged by the respective metalloligand (Figure 6). This is E


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some double bond character between the nitrogen atoms and the C5H4 ring to which they are connected.

Here again, the orientations of the Mo−ringcentroid axis with respect to the corresponding Au−Au edge differ remarkably between 21 and 22 (Figure 9) and, consequently, the bonding

Figure 9. Schematic representations of 21 (left) and 22 (right).

modes of the carbonyl ligands (see Supporting Information). This is well-reflected by the values of the CCp−Mo−Csquare angle β reported in Table 3. Figure 7. View of the crystal structure of 21. Selected bond lengths (Å) and angles (deg): Au1−Au2 2.7598(5), Au2−Au3 2.8248(5), Mo1− Au1 2.8437(7), Mo1−Au2 2.8140(8), Mo2−Au2 2.7753(8), Mo2−Au3 2.8114(7); Au2−Au1−Au2′ 93.51(2), Au1−Au2−Au3 87.87(2), Au2− Au3−Au2′ 90.74(2), Mo1′−Au1−Mo1 149.88(3), Mo1−Au2−Mo2 155.20(2), Mo2−Au3−Mo2′ 162.07(3).

Table 3. Values of the Angle β for 21 and 22a β

monoclinic 21

orthorhombic 22

C1Cp−Mo1−Csquare C2Cp−Mo2−Csquare C3Cp−Mo1−Csquare C4Cp−Mo2−Csquare

128.24(2) 137.34(2) 134.08(4) 137.34(2)

133.10(4) 137.05(4)

a 1 C Cp, C2Cp, C1′Cp, C2′Cp, C3Cp, and C4Cp are the centroids of the aromatic rings (CCp), and Csquare is the centroid of the Au4 square.

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DISCUSSION Some heterometallic clusters with a central copper, silver, or gold triangle displaying d10−d10 separations consistent with significant metallophilic interactions have been structurally characterized2q and include [M{m}]3 clusters with a ν2-triangular metal core (M = Cu, m = Fe19 and Os;25 M = Ag, m = Nb,27 Ta,27 Co,28 and Rh;29 M = Au, m = Nb).30 Even fewer octanuclear heterometallic clusters [M{m}]4 with a central M4 square or rectangle formed by group 11 metals separated by less than 3.4 Å have been characterized, and their edge-bridging metalloligands {m} form an external square (or rectangle) circumscribing the inner one, thus generating more or less distorted square-in-a-square-type structures.2q Two of these clusters feature Cu(I) or Ag(I) squares, with edge-bridging Co(CO)4 metalloligands,26 and two other examples consisted of Ag(I) and Au(I) squares, edgebridged by a Fe(CO)4 metalloligand.20,21 The first examples of edge-bridging 17-electron metal carbonyl moieties were encountered with the M1Cp(CO)3 (M1 = Cr, Mo, and W) metalloligands in Pd2M12 and Pt2M12 centrosymmetric clusters related to 10 and 11.24 Considering these bridging groups as

Figure 8. Perspective views of the Au4Mo4 metal core of 21.

gold square of 21 (2.792(1) Å) and 22 (2.774(1) Å) are significantly shorter than that in [Au4{Fe(CO)4}4]4− (2.902(2) Å).20 The molybdenum atoms are not coplanar with the inner silver square, and slight tetrahedral distortions about the silver atoms result in two opposite Mo atoms to be located on the same side of the Au4 plane. The bonding of the carbonyl ligands is analyzed in Table S5. As observed for 17 and 19, the planar environment of the N atoms and the values of the N−C bonds are indicative of F


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Organometallics

metal core geometry, a square (or rectangle) for Ag(I) and Au(I), and a triangle for Cu(I). However, whereas a silver square was obtained with the Co(CO)4 metalloligand,26b neither a corresponding Au4Co4 square cluster nor a Cu3Co3 triangular cluster have been isolated to the best of our knowledge. It is noteworthy that a Cu4Co4 square-in-a-square cluster similar to the Ag4Co4 cluster was obtained from analogous reactions.26a Copper thus appears to be the only group 11 metal to give rise in metal carbonyl chemistry to both ν2-triangular and square-in-asquare structures, although with different bridging metalloligands. The average M−Mo metal−metal distances in 17 (M = Cu, 2.647(1) Å), 19 (M = Ag, 2.847(1) Å), and 21 (M = Au, 2.811(1) Å) may be directly compared since they involve the same metalloligand Mo(η5-C5H4NMe2)(CO)3; they reflect the changes in the size of the atom M.38a The average d10−d10 distances in 17, 19, and 21, the different metal radii (atomic,37 covalent,38 and van der Waals),39 and the M−M distances in the bulk metals are reported in Table 4 for comparison. The average M−M distances in 17, 19, and 21 are closer to the sum of the estimated covalent radii for four-coordinated Cu(I), Ag(I) and Au(I) centers38c than to the sum of the covalent radii for two-coordinated univalent group 11 centers.38b,c This could therefore suggest that the environment around each d10 center is closer to four-coordinated than to two-coordinated. Indeed, the sum of the covalent radii is 2.58, 2.92, and 2.74 Å for fourcoordinate Cu(I), Ag(I), and Au(I), respectively, and 2.26, 2.66, and 2.50 Å for two-coordinate Cu(I), Ag(I), and Au(I), respectively. The average d10−d10 distances in 17, 19, and 21 of 2.617, 2.869, and 2.793 Å, respectively, are also close to the metal−metal distances in the bulk metals, and to the sum of the atomic radii of the group 11 elements. These values further confirm that Au(I) is smaller than Ag(I), as a result of relativistic effects.3b,4s,40 The Cu−Cu and Mo−Cu distances observed in our clusters are generally shorter than those found in the literature (average values from Cambridge Structural Database 2016: 2.713 Å, for 7175 distances from 2754 structures and 2.717 Å for 469 distances from 170 structures, respectively; Figure S4). The same applies to the Ag−Ag and Mo−Ag distances (average values 3.038 Å, for 9607 distances from 2073 structures and 2.945 Å for 105 distances from 50 structures, respectively; Figure S5) and to the Au−Au and Mo−Au distances (average value of 2.936 Å for 6774 distances from 1578 structures and 2.824 Å for 129 distances from 50 structures, respectively; Figure S6).

formally 3-electron donors allowed us to easily rationalize the structures of these clusters and led to a 16-electron environment for the Pd and Pt centers, respectively.24,31 This concept was extended to isoelectronic metalloligands such as Co(CO)4, Co(CO)3L, Fe(CO)2(NO)L, Mn(CO)4L, and MoCp(CO)2L (L = 2e donor).32 Furthermore, a unique example of triply bridging MoCp(CO)3 moiety acting formally as a 5-electron donor was discovered in a cubane-type Pd3Mo cluster.33 Only few heterometallic clusters with either a triangular or a square metal core have been reported for d10 ions of the noncoinage metals, Pd(0) or Pt(0)34 and Cd(II) or Hg(II),35 respectively, or in other cases.36 In 19 and 21, attractive d10−d10 interactions appear responsible for the inward bending of the M′−M−M′ sequence and result in a star-shaped octanuclear metal core, as was observed for 20 and 22.13 The synthesis of these clusters can be formally viewed as generating a neutral heterodinuclear M{m} entity that oligomerizes to provide the d10 metal center M with a suitable coordination environment. Why are these cyclic oligomers [M{m}]n different for M = Cu (n = 3) and M = Ag or Au (n = 4)? From a comparison with relevant clusters reported in the literature, one can ask the question whether the nature of the d10 metal and associated d10−d10 interactions can result in a preference for a ν2-triangular or square-in-a-square core geometry in such mixed-metal clusters (Scheme 3). Scheme 3. ν2-Triangular or Square-in-a-Square Core Geometries

Squares versus Triangles. Complexes 17−22 represent the first series of heterobimetallic clusters containing group 6 and group 11 transition metal centers and displaying d10−d10 interactions. The triangular, trianionic cluster in [NEt4]3[Cu3{Fe(CO)4}3] (23),19 and the tetraanionic clusters in [NMe3CH 2 Ph] 4 [Ag 4 {Fe(CO) 4 } 4 ] (24) 20 and [NEt 4 ] 4 [Au 4 {Fe(CO)4}4] (25),21 in which each Cu−Cu, Ag−Ag or Au−Au edge is bridged by a Fe(CO)4 metalloligand (Scheme 4), represent the only other homologous series of oligomeric clusters of

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CONCLUSIONS A new family of heterometallic clusters of the general formula [M−Mo(η5-C5H4NMe2)(CO)3]n (M = Cu, Ag, and Au) has been obtained by reaction between the tricarbonylmolybdate reagent [Mo(η5-C5H4NMe2)(CO)3]− and [Cu(NCMe)4]BF4, AgBF4, or [N(n-Bu)4][AuBr2], respectively. The metalloligand Mo(η5-C5H4NMe2)(CO)3 is bonded to the coinage metals, through direct metal−metal bonding and bridging carbonyls, leading to 2D raft-type skeletons featuring frequency 2 (ν2) M3 triangular or M4 square-in-a-square-type structures. The bare d10 centers M in these clusters develop attractive intramolecular d10−d10 interactions that are consistent with the metrical data obtained by X-ray diffraction. Whereas in the case of Cu, the cluster core forms a triangle, for M = Ag or Au, a M4 square is formed. This may be due to the possibility to generate additional, weaker metallophilic interactions across the diagonal of square structures (Ag1···Ag3 3.943(1), Ag2···Ag2′ 4.1675(7); Au1··· Au3 3.875(1), Au2···Au2′ 4.0205(7)).13 Although metalloligand

Scheme 4. Triangular and Square-in-a-Square-Type Clusters with the Fe(CO)4 Metalloligand

the type [M{m}]n with the same bridging metalloligand and they provide an interesting parallel with 17−22. With the Fe(CO)4, MoCp(CO)3, and Mo(η5-C5H4NMe2)(CO)3 bridging metalloligands, a given d10 ion leads to the same G


Article

Organometallics Table 4. Covalent, Atomic, Metal, and van der Waals Radii and Experimental M−M Distances (in Å) experimental M−M average values for [M−MoCp(CO)3]n experimental M−M average values for [M−Mo(η5−C5H4NMe2)(CO)3]n 2 × covalent radius38a 2 × covalent radius (two-coordinated univalent M)38b,c 2 × covalent radius (four-coordinated univalent M)38c 2 × atomic radius37 2 × metal atom radius37 2 × van der Waals radius39

M = Ag

M = Au

2.870(1) 2.869(1) 2.90(5) 2.66 2.92 2.678 2.89 3.44

2.774(1) 2.792(1) 2.72(6) 2.50 2.74 2.672 2.884 3.32

Synthesis of [Cu3{Mo(η5-C5H4NMe2)(CO)3}3] (17). Solid [Cu(NCMe)4]BF4 (0.103 g, 0.33 mmol) was added to a suspension of Li·5·2DME (0.156 g, 0.33 mmol) in toluene at −20 °C. The temperature was raised to room temperature and the reaction mixture was stirred for 20 min, then filtered, and the solution was concentrated under reduced pressure. Precipitation with pentane afforded pale yellow 17, which was washed with pentane (3 × 10 mL) (0.042 g, 36%). Yellow crystals of 17·1.5C7H8 were obtained from a toluene/pentane mixture. Anal. Calcd for C30H30Cu3Mo3N3O9: C, 34.15; H, 2.87; N, 3.98. Found: C, 33.41; H, 2.52; N, 4.31. Despite numerous attempts, no better elemental analyses could be obtained. IR (KBr) ν(CO) = 1957w, 1860s br, 1812s br cm−1. 1H NMR (C6D6) AA’XX’ spin system, δ 4.70 and 4.24 (pseudo triplets, 12H, Cp), 2.19 (s, 18H, N(CH3)2). 13 C{1H} NMR (C6D6) 224.3 (CO), 82.9 (Cp), 67.0 (Cp), 39.6 (N(CH3)2. The resonance of Cipso was not observed. Synthesis of [Ag4{Mo(η5-C5H4NMe2)(CO)3}4] (19). Solid AgBF4 (0.115 g, 0.59 mmol) was added to a stirred suspension of Li·5·2DME (0.28 g, 0.59 mmol) in toluene at −20 °C in the dark. The temperature was raised to room temperature, and the mixture was stirred for 20 min. The resulting solution was filtered, and the solution was concentrated under reduced pressure. Precipitation with pentane afforded pale yellow 19, which was filtered and washed with pentane (3 × 10 mL) (0.074 g, 32%). Pale yellow, light-sensitive crystals of 19 were obtained from toluene/pentane. Anal. Calcd for C40H40Ag4Mo4N4O12: C, 30.33; H, 2.55; N, 3.54. Found: C, 30.54; H, 2.62; N, 3.92. IR (KBr) ν(CO) = 1931s, 1881s, 1850s sh cm−1. IR (CH2Cl2) ν(CO) = 1934s, 1885s, 1854s cm−1. 1H NMR (C6D6) AA’XX’ spin system, δ 4.85 and 4.70 (pseudo triplets, 16H, Cp), 2.25 (s, 24H, N(CH3)2). 13C{1H} NMR (C6D6) 80.2 (Cp), 67.3 (Cp), 39.9 (N(CH3)2. The resonance of Cipso was not observed. Synthesis of [Au4{Mo(η5-C5H4NMe2)(CO)3}4] (21). Solid [N(nBu)4][AuBr2] (0.128 g, 0.21 mmol) was added to a suspension of Li·5· 2DME (0.102 g, 0.21 mmol) in toluene at −40 °C. After it was stirred for 15 min, the red solution was filtered, and the solution was concentrated under reduced pressure. Precipitation with cold pentane afforded red 21, which was collected by filtration and washed with cold pentane. This complex is very temperature sensitive in the solid state and in solution and decomposes within a few minutes at room temperature, which prevented collection of NMR data and satisfactory elemental analysis. Red crystals of 21 were obtained at −30 °C from toluene/pentane. IR (THF) ν(CO) = 1929s br, 1825s br cm−1. IR (KBr) ν(CO) = 1928s, 1899s, 1873s br cm−1. Crystal Structure Determinations. The data were collected on a Nonius Kappa-CCD area detector diffractometer (Mo Kα, λ = 0.71070 Å, phi scan). The relevant data are summarized in Table S6. The cell parameters were determined from reflections taken from one set of 10 frames (1.0° steps in phi angle), each at 20 s exposure. The structures were solved using direct methods (SHELXS97) and refined against F2 using the SHELXL97 software.43 The absorption was corrected empirically (with Sortav) for the area detector data. All non-hydrogen atoms were refined with anisotropic parameters. The hydrogen atoms were included in their calculated positions and refined with a riding model in SHELXL97. The crystallographic information files have been deposited with the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K., under deposition numbers 1503270− 1503272.

Mo(η5-C5H4NMe2)(CO)3 was shown to behave as a bridging ligand in Mo−Pt complex 16 and is isoelectronic to 9 which showed some bridging character in 15, it only interacted in the complexes reported in this work with the d10 ions. These complexes are the first heterometallic clusters with such geometries involving group 6 and group 11 transition metals. From the relevant metal carbonyl clusters reported in the literature, it appears that Cu(I) is so far the only coinage metal able to form either triangular or square cores, although with different but related metalloligands. The anionic linear chain complexes [M1−M−M1]− (2−4) have been previously shown to react with one equiv of cation M+ from suitable d10 metal precursors to give the corresponding hexa- or octanuclear mixed-metal clusters 18, 20, and 22, respectively.8 In turn, the latter clusters can be converted back to heterotrinuclear chain complexes 2−4 upon addition of 1 equiv of carbonylmetalate. Such versatile and controlled access to various heterometallic clusters opens interesting perspectives for the incorporation of different d10 metal centers within the same cluster.8 Previous theoretical calculations performed on the series of related Cp-containing carbonyl clusters analyzed the reasons for a preferred geometry as a function of M and of the stabilizing influence of the d10−d10 interactions and concluded that diagonal interactions in the square structures contribute to an additional stabilization.13 It is noteworthy that for a given group 11 metal only one cyclic oligomer (n = 3 or 4) was observed, and we have no indication of equilibria in solution involving triangles or squares, although this could be conceivable with only one form crystallizing for solubility reasons and compatible with the reactivity of these clusters.8 This is different from the situations recently observed in some metallo-supramolecular complexes where reversible metal−ligand bond formation (no metal−metal bonding was involved) accounted for the occurrence of equilibria between triangular and square structures.41

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M = Cu 2.627(8) 2.617(1) 2.64(4) 2.26 2.58 2.346 2.556 2.80

EXPERIMENTAL SECTION

General Procedures. Reactions were carried out under an atmosphere of nitrogen by means of conventional Schlenk techniques. Solvents were dried according to standard procedures. Elemental analyses were performed by the Service de Microanalyses, Université de Strasbourg. NMR spectra were recorded on a Bruker Avance 300 instrument (1H, 300 MHz; 13C{1H}, 75.47 MHz). Chemical shifts (in ppm) were measured at ambient temperature and are referenced to TMS. Assignments are based on APT and DEPT spectra and 1H, 1 H−COSY and 1H, 13C-HMQC experiments. IR spectra were recorded in the region 4000−400 cm−1 on a FT-IR IFS66 Bruker spectrometer. Li[Mo(η5-C5H4NMe2)(CO)3]·2DME (Li·5·2DME) was synthesized according to literature procedure.14 Compounds [Cu(NCMe)4]PF6 and AgBF4 were purchased from Strem Chemicals and Sigma-Aldrich, respectively, and [N(n-Bu)4][AuBr2] was synthesized according to literature procedures.42 H


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Organometallics

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00745. ORTEP views, histograms of metal−metal distances from the CSD, bond distances and angles, analysis of the bonding of the carbonyl ligands, table of crystallographic data, data collection parameters and refinement results. (PDF) Crystallographic information files for 17, 19, and 21 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pierre Braunstein: 0000-0002-4377-604X Present Address

R.W.: Université de Strasbourg, CNRS, IBMP UPR 2357, 12 rue du Général Zimmer, 67000 Strasbourg, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the CNRS, the Ministère de l′Enseignement Supérieur et de la Recherche, the DFH/UFA, the DFG International Research Training Group GRK532 and the Agence Nationale de la Recherche (ANR-06-BLAN-410). We are grateful to Drs. L. Karmazin and A. DeCian for the collection of the X-ray data. Dedicated to Prof. Antonio Tiripicchio on the occasion of his 80th birthday and Profs. Rick Adams and Carlo Mealli on the occasion of their 70th birthday and in recognition of their numerous outstanding contributions to the field of cluster chemistry.

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REFERENCES

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