Ligated Decanuclear Tl–Pd Cluster, Pd - ACS Publications

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High-Yield Synthesis of PPh3-Ligated Decanuclear Tl−Pd Cluster, Pd9[Tl(acac)](CO)9(PPh3)6: Comparative Analysis of Tl(I)−Pd(0) Bonding Connectivities with Known Tl−Pd Clusters and Resulting Insight Concerning Their Dissimilar Dynamic Solution Behavior Evgueni G. Mednikov,* Nicky Vo, Charles G. Fry, and Lawrence F. Dahl* Department of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: The new Tl(I)−Pd(0) cluster Pd9[μ3/3-Tl(acac)](μ2-CO)6(μ3-CO)3(PPh3)6 (1) was prepared in high yields (over 90%), both by reaction of Pd10(CO)12(PPh3)6 (4), PPh3, and TlPF6 in THF in the presence of acetylacetone (Hacac) and base (NEt 3 ) and by direct reaction of Pd10(CO)12(PPh3)6 with PPh3 and Tl(acac). The composition and molecular structure of 1 were unambiguously established from 100 K CCD X-ray diffractometry studies of two solvated crystals, 1·1.5Hacac·0.5THF (1A) and 1·0.3THF (1B), which showed essentially identical geometries for the entire Pd9Tl(CO)9P6 fragment of pseudo-C3v symmetry; its composition is in agreement with X-ray Tl/Pd field-emission microanalysis with a scanning electron microscope for crystals of 1B. This cluster can be viewed as a markedly deformed Pd6 octahedron (oct) with the three Pd(oct) atoms of one of its eight triangular faces connected both by three edge-bridging wingtip (wt) Pd(μ2-CO)2PPh3 fragments and by a symmetrical capping Tl(I). Three triply bridging carbonyl ligands asymmetrically cap the lower alternate 3-fold-related triangular faces of the Pd6 octahedron, and the three other PPh3 ligands are each coordinated to Pd atoms in the geometrically opposite staggered Pd(oct)3 face. The 6s25d10 Tl(I) is also equivalently attached to both chelating O atoms of a bidentate acetylacetonate (acac) monoanion. Although the C2 axis of the pseudo-C2v planar Tl(acac) fragment is approximately parallel to the pseudo-C3 axis of the TlPd9 core, the orientation of the Tl(acac) plane relative to the octahedral-based Pd9 geometry is considerably different for each of the three independent nondisordered molecules of 1 in 1A and 1B; these different planar Tl(acac) orientations may be mainly attributed to anisotropic crystal-packing effects. Coordination of the Tl(I) atom to the three Pd(oct) atoms of the Pd9 core presumably occurs via its so-called “inert” 6s2 electron pair with resulting three short Tl−Pd(oct) connectivities of mean distance 2.83 Å; these connectivities together with three longer Tl−Pd(wt) ones of mean distance 3.15 Å give rise to a (crown-like)Pd6 sextuple (μ3/3-Tl) coordination mode. Of particular stereochemical interest is a comparison of solution behavior of 1 with that for the known structurally related analogue, Pd9[μ3-TlCo(CO)3L](μ2-CO)6(μ3-CO)3L6 (2) (with L = PEt3 instead of PPh3). In 2 the Tl(I) is alternatively attached to a trigonal-bipyramidal Co(CO)3L monoanion and primarily coordinated to the three inner Pd(oct) atoms of a similar PR3/CO-ligated octahedron; corresponding Tl−Pd(oct) and Tl−Pd(wt) mean distances for two independent molecules in 2 are 2.77 and 3.31 Å, respectively. Variable-temperature 31P{1H} NMR solution data of 1 indicate the occurrence of presumed fast wobbling-like motion of the [μ3/3-Tl(acac)] entity about the pseudo-C3 axis of the Pd9(μ2-CO)6(μ3CO)3P6 fragment without Pd−Tl detachment (i.e., the entire cluster of 1 remains intact). In direct contrast, corresponding temperature-dependent 31P and 13C NMR data of 2 instead are consistent with rapid, reversible dissociation/association of the entire [μ3-TlCo(CO)3L] ligand from the analogous Pd9(μ2-CO)6(μ3-CO)3P6 fragment of 2. This highly dissimilar dynamic solution behavior that points to a stronger Tl(I) attachment to the Pd9 core in 1 than that in 2 may be attributed from the above crystallographic evidence to greater involvement of the outer three wingtip Pd(wt) atoms in bonding connectivities to the Tl(I) in 1 compared to predominant bonding connectivities of only the three inner Pd(oct) atoms to the Tl(I) in 2. 1H NMR solution spectra of 1 also suggest significant covalent character in the bidentate Tl−O(acac) bonding in 1 based upon the observation of H(acac)−Tl coupling; this premise is consistent with its Tl−O distances of 2.35 Å (av) being ca. 0.2 Å shorter than those of 2.52 Å (av) found in crystalline Tl(acac), which with no observed H−Tl NMR coupling in solution implies ionicity of its bidentate continued...

Special Issue: F. Gordon A. Stone Commemorative Issue Received: November 17, 2011 Published: March 5, 2012 © 2012 American Chemical Society

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Organometallics

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Tl−O bonding. Both 1 and 2 conform to an 86 CVE count expected for an octahedral metal polyhedron based upon the Tl(I) and each wingtip Pd(μ2-CO)2L fragment contributing 2 and 4 CVEs, respectively.

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INTRODUCTION Our initial efforts to obtain Au−Pd clusters from Pdn(CO)x(PEt3)y precursors and Au(I) chloride complexes in the presence of TlPF6 (as a Cl− scavenger) gave a completely unexpected product. Instead of activation of the gold precursor via elimination of chloride as insoluble TlCl, alternatively thallium(I) was incorporated into the resulting heterometallic core to give th e first repo rt ed Pd−Tl cluster, [Pd12Tl2(CO)9(PEt3)9]2+ dication (3),1a whose geometrical unprecedented pseudo-C3h Tl2Pd12 core may be viewed as Pd(ax)−Pd(eq) edge fusions of three Pd5 trigonal bipyramids to a central Tl2Pd3 trigonal bipyramid. It was also shown that 3 is a product of the spontaneous conversion of the [(μ6Tl)Pd6(CO)6(PEt3)6]+ sandwich cluster (as the [PF6]− salt), which was obtained along with the isomorphous crystalline [TlPt6]+ analogue in high yields.1b The third known Tl−Pd cluster is Pd9[TlCo(CO)3(PEt3)](CO)9(PEt3)6 (2),1c which is structurally related (vide inf ra) to Pd9[Tl(acac)](CO)9(PPh3)6 (1) reported herein. Although the current Cambridge Crystallographic database2 contains no other Pd−Tl clusters possessing metal polyhedra, it lists a number of Pd−Tl complexes: these include dinuclear ones, Pd(0)−Tl(I),3a Pd(II)−Tl(I),3b and Pd(II)−Tl(III),3b and ones with tri- and pentanuclear metal chains, Pd(0)−Tl(I)−Pd(0),3c Pd(0)− Tl(I)−Au(I),3d and Pd(II)−Tl(I)−Pd(II)−Tl(I)−Pd(II).3e It has been experimentally shown in CO/PR3-ligated palladium homo-/heterometallic clusters that the steric/ electronic influence of different phosphine R-substituents (aside from the gas-phase composition) can be a dominant factor in controlling the self-assembly of metal atoms into a particular architecture.4 Palladium distinguishes itself from the other transition metal elements, especially its group 10 congeneric Ni and Pt metals, in forming relatively weak metal−metal and metal−CO connectivities, which are responsible for both the unparalleled variety of close-packed structural types of nanosized Pdn(CO)x(PR3)y clusters and their facile lability to undergo geometrical changes via chemical reactions.4 In this connection, the three previously known Tl−Pd clusters (mentioned above) have the same PEt3 ligand. This publication presents the high-yield synthesis of the new Tl−Pd cluster Pd9[μ3/3-Tl(acac)](CO)9(PPh3)6 (1), which is stabilized by triphenylphosphine ligands. The overall geometrical features of this PPh3-ligated cluster are closely similar to those previously found in the Pd9[TlCo(CO)3L)](CO)9L6 cluster (2), in which L = PEt3, instead of PPh3, thereby indicating a particular adaptive role of the common octahedralbased Pd9(CO)9L6 composite to an electron-pair capping linkage involving the “inert” 6s2 electron pair of Tl(I) and the resulting expected conformity of both 1 and 2 to an octahedral 86 CVE (cluster valence electron) count. In spite of this overall similarity, the additional interactions (indicated crystallographically) of the Tl(I) with the three wingtip Pd(wt) atoms in the PPh3-ligated cluster 1 are apparently responsible for the remarkable change in dynamic solution behavior of 1 relative to that of 2. The overall changes in the bonding connectivities of Tl(I) from primarily μ3-Tl in 2,1c to intermediate μ3/3-Tl in 1, and to the ultimate μ6-Tl found for each similarly coordinated Tl(I) in [(μ6-Tl)2Pd12(CO)9(PEt3)9]2+ 1a are consistent with

their NMR data in solution. Thus, this research emphasizes the general importance in correlating the spectroscopically deduced dynamic properties of geometrically related clusters in solution (especially, if different) with their static structure/bonding properties in the crystalline state.

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RESULTS AND DISCUSSION Synthesis and Characterization. Pd 9 [Tl(acac)](CO)9(PPh3)6 (1) was obtained from Pd10(CO)12(PPh3)6 (4) either by reaction with TlPF6 in THF in the presence of acetylacetone (Hacac) under basic conditions (eqs 1,2) or by direct reaction with Tl(acac) under neutral conditions (eq 3). 9Pd10(CO)12 L6 + 6L + 10TlPF6 + 10Hacac + 10Na2CO3 → 10Pd 9[Tl(acac)](CO)9 L6 + 10NaHCO3 + 10NaPF6 + 18CO

(1)

9Pd10(CO)12 L6 + 6L + 10TlPF6 + 10Hacac + 10NEt3 → 10Pd 9[Tl(acac)](CO)9 L6 + 10[HNEt3][PF6] + 18CO

(2)

9Pd10(CO)12 L6 + 6L + 10Tl(acac) → 10Pd 9[Tl(acac)](CO)9 L6 + 18CO

(3)

where L = PPh3. The highest yields of 1 (up to 94%) were achieved in reactions 2 and 3 with amounts of PPh3 and Tl(I) salts being 1.1 equiv relative to that required by the composition of 1. If 2 or 4 equiv of Tl(I) were used, resulting yields of 1 dropped to 67% and 13%, respectively. None of the co-products in these reactions were sufficiently stable to be crystallized. This cluster, 1, is appreciably soluble in CH2Cl2, CHCl3, and THF, but poorly soluble in toluene. However, its solubility in toluene could be greatly increased by an initial dissolution in any of the three earlier mentioned solvents, followed by evaporation and redissolving of the residue in toluene. Solutions of 1 are somewhat unstable. For example, 17% of 1 (from 31P{1H} NMR estimation) had decomposed after storage of its CDCl3 solution for one week at −20 °C, whereas virtually the same amount (18%) of 1 had decomposed after storage of its toluene-d8 solution for one day at room temperature. Infrared spectra confirmed the presence of doubly and triply bonded CO ligands and a Tl(I)-chelated acac ligand in 1. Although solid-state (Nujol) IR spectra of 1 varied in intensity and sometimes in the number of resolved bands, spectra in CHCl3 solutions (and likewise in THF) obtained for the same samples were always uniform, with carbonyl frequencies at 1906−1902 vs, 1845−1840 w-m, 1813−1809 m cm−1 and acac bands at 1580−1577 m, 1512 w, 1481−1479 w, 1435 m-s cm−1 in accordance with the molecular structure of 1 established from the X-ray diffraction determinations. (See Supporting Information for recorded IR data and possible sources for the observed variations in the solid-state IR spectra.) 31 1 P{ H} NMR solution spectra (independent of solvent used) displayed two sets of doublets with an integral ratio of 2879

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Organometallics

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Table 1. Comparison of Relevant Interatomic Distances for the Pd9Tl(CO)9L6 Fragments in the Two Independent Molecules of Pd9[μ3-Tl(acac)](μ2-CO)6(μ3-CO)3L6 (1) in 1·1.5Hacac·0.5THF (1A), the Nondisordered Molecule of 1 in 1·0.3THF (1B), L = PPh3, and Pd9[μ3-TlCo(CO)3L](μ2-CO)6(μ3-CO)3L6, L = PEt3, (2) 1st and 2nd molecules of 1 in 1A

Pd9[TlCo(CO)3L](CO)9L6 (2)c

nondisordered molecule of 1 in 1B

Na,b

mean [Å]

range [Å]

mean [Å]

mean [Å]

range [Å]

Tl−Pd(A)

3

2.825(1)−2.859(1)

2.77

2.747(1)−2.798(1)

3

3.18

3.123(1)−3.256(1)

3.31

3.197(1)−3.433(1)

Pd(A)−Pd(C)

6

2.72

2.705(1)−2.728(1)

2.72

2.694(1)−2.733(1)

Pd(A)−Pd(A)

3

3.03

3.022(1)−3.036(1)

3.06

3.017(1)−3.089(1)

Pd(A)−Pd(B)

6

2.76

2.724(1)−2.807(1)

2.76

2.734(1)−2.788(1)

Pd(B)−Pd(B)

3

2.82

2.787(1)−2.869(1)

2.81

2.774(1)−2.821(1)

Pd(B)−P(B)

3

2.33

2.321(3)−2.335(2)

2.31

2.301(2)−2.329(2)

Pd(C)−P(C)

3

2.32

2.319(3)−2.326(3)

2.32

2.305(2)−2.331(2)

Pd(A)−(μ2-CO)

6

2.09

2.062(10)−2.148(10)

2.09

2.070(6)−2.106(6)

Pd(C)−(μ2-CO)

6

2.03

2.008(11)−2.045(10)

2.03

2.022(5)−2.051(5)

Pd(A)−(μ3-CO)

3

2.32

2.269(10)−2.383(10)

2.32

2.262(5)−2.361(5)

Pd(B)−(μ3-CO)

6

2.11

2.084(10)−2.161(9)

2.12

2.097(5)−2.144(5)

μ2-C−O

6

1.15

1.139(12)−1.155(12)

1.16

1.148(6)−1.167(6)

μ3-C−O

3

2.818(1)−2.877(1) 2.798(1)−2.853(1) 2.973(1)−3.230(1) 2.981(1)−3.289(1) 2.695(1)−2.734(1) 2.688(1)−2.755(1) 3.026(1)−3.070(1) 3.021(1)−3.073(1) 2.727(1)−2.759(1) 2.721(1)−2.796(1) 2.796(1)−2.851(1) 2.787(1)−2.840(1) 2.319(2)−2.320(2) 2.314(2)−2.339(2) 2.302(2)−2.327(2) 2.324(2)−2.326(2) 2.061(9)−2.120(8) 2.046(9)−2.139(9) 2.020(8)−2.078(9) 2.023(8)−2.044(8) 2.291(9)−2.330(8) 2.304(9)−2.371(9) 2.104(9)−2.130(8) 2.108(8)−2.128(8) 1.125(11)−1.168(10) 1.137(11)−1.178(11) 1.151(10)−1.162(10) 1.136(10)−1.163(10)

2.84

Tl···Pd(C)

2.84 2.82 3.14 3.13 2.71 2.73 3.05 3.04 2.75 2.76 2.82 2.82 2.32 2.32 2.32 2.33 2.09 2.09 2.04 2.03 2.31 2.33 2.12 2.12 1.15 1.15 1.16 1.15

1.18

1.169(12)−1.179(12)

1.17

1.158(6)−1.175(6)

bonding connectivity

range [Å]

a

N denotes the number of symmetry-equivalent connectivities. bAll connectivities are given for the pseudo-C3v symmetry of Pd9Tl(CO)9L6 fragments, L = PPh3 (1), PEt3 (2). Coordination of acac to Tl(I) in the Pd9Tl(CO)9(PPh3)6 fragment reduces the symmetry from pseudo-C3v to C1. c Averaged for two independent molecules of 2.

Table 2. Selected Mean Interatomic Distances [Å] and Angles [deg] for the [μ3-Tl(acac)] Fragment in the Two Independent Molecules of Pd9[μ3-Tl(acac)](μ2-CO)6(μ3-CO)3(PPh3)6 (1) in 1·1.5Hacac·0.5THF (1A) and in the Nondisordered Molecule of 1 in 1·0.3THF (1B) under Pseudo-C2v Symmetry 1st and 2nd molecules of 1 in 1A bonding connectivity, angle

a

a

N

Tl−O

2

C−O

2

C−CH

2

O−Tl−O

1

C−O−Tl

2

mean 2.35 2.33 1.27 1.27 1.40 1.38 78.119(4) 79.540(4) 130.8 129.5

range 2.330(6)−2.368(7) 2.304(7)−2.347(6) 1.261(12) −1.276(11) 1.267(11)−1.275(12) 1.392(15)−1.401(14) 1.375(15)− 1.394(15)

nondisordered molecule of 1 in 1B mean

range

2.36

2.322(7)− 2.392(8)

1.27

1.259(14)−1.284(14)

1.39

1.394(18)−1.395(17)

77.657(3) 129.906(5)−131.605(5) 128.787(5)−130.128(5)

130.8

129.850(5)−131.808(5)

N denotes the number of symmetry-equivalent connectivities/angles under C2v symmetry of the Tl(acac) fragment. 31

1:1, in agreement with the solid-state molecular structure. (See Supporting Information for recorded spectra.) We assigned the doublet at 17.17 ppm (2,3JP(C)‑Tl = 333 Hz) to the Pd(C)attached P(C) atoms and the doublet at 14.88 ppm (3JP(B)‑Tl = 148 Hz) to the Pd(B)-attached P(B) atoms (in CDCl3). The assignment of these two signals is based on the expectation that the 31P(C)−Tl coupling would be larger than that of the

P(B)−Tl. In fact, the P(C) atoms are much closer to Tl than the P(B) atoms, with the P(B) atoms being separated from Tl by three bonds (3J coupling), whereas P(C) atoms have both two-bond (2J) and three-bond (3J) couplings to Tl. Because the difference in chemical shifts of 278 Hz between the P(C) and P(B) signals and their 31P−203,205Tl spin−spin splitting (ca. 140−340 Hz) potentially overlapped each other, the correct2880

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Organometallics

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Figure 1. (a and b) Side views of the Pd9[Tl(acac)]P6 fragment and the entire Pd9[μ3/3-Tl(acac)](μ2-CO)6(μ3-CO)3(PPh3)6 (1) geometry (without Ph substituents), respectively, for the first independent molecule of 1 in 1·1.5Hacac·0.5THF (1A). (c and d) Top views of the Pd9[Tl(acac)]P6 fragments for the first and second nondisordered independent molecules of 1 in 1A, respectively, showing the different orientation of the pseudo-C2v planar Tl(acac) fragment (acac is drawn in black) in each molecule of 1 relative to the pseudo-C3 axis of the Pd9Tl metal core (dashed lines are omitted for clarity). These latter two views may be considered to represent two snapshots of the dynamic behavior of 1 in solution; the transformation between these two orientations is presumed to occur via a low-energy wobbling-like migration of the planar C2v Tl(acac) fragment about the pseudo-C3 axis of the Pd9Tl metal core.

9.7%)/(90.6−90.3%)] obtained for the four crystals of 1B from use of Pd L and Tl M lines. a. Solid-State Molecular Structure of Pd9[Tl(acac)](CO)9(PPh3)6 (1). Pd9Tl(CO)9P6 Fragment. Pd9[Tl(acac)](CO)9(PPh3)6 (1) crystallizes with different numbers of solvent molecules, depending on the reaction conditions. Lowtemperature X-ray crystal structures of 1 were determined for two crystals obtained from different synthetic procedures, namely, 1·1.5Hacac·0.5THF (1A) and 1·0.3THF (1B). The asymmetric part of the centrosymmetric triclinic unit cell (space group P1̅) found in each structure of 1A and 1B consists of two independent molecules of Pd9[Tl(acac)](CO)9(PPh3)6 (1) as well as solvent molecules in general positions. Since the second molecule of 1 in the crystal structure of 1B is

ness of the above assignment of doublets was ascertained from 31 P(B)−203,205Tl and 31P(C)− 203,205Tl saturation experiments. Details of these experiments as well as recorded 31P{1H} NMR spectra for other solvents (viz., THF-d8, CD2Cl2 and toluened8) are given in the Supporting Information. X-ray Tl/Pd field-emission microanalysis (with a scanning electron microscope, LEO 1530 FESEM) was carried out on four crystals of 1B. (See Supporting Information for the recorded SEM spectra along with details of analysis.) The four SEM spectra are virtually indistinguishable from one another, and the Tl/Pd-atom % ratio of 10%/90% established for the TlPd9 core of 1 from the single-crystal X-ray studies (vide inf ra) is in good agreement with that of 9.5%/90.5% [range (9.4− 2881

dx.doi.org/10.1021/om201150x | Organometallics 2012, 31, 2878−2886

Organometallics

Article

Comparative Analysis of the Tl(I) Connectivities in 1 with Tl(I) Connectivities in the Structurally Related Pd9[TlCo(CO)3(PEt3)](CO)9(PEt3)6 (2) and [Pd12Tl2(CO)9(PEt3)9]2+ Dication (3). The Pd9(CO)9L6 fragment in 21c is geometrically analogous to that in 1.7 However, in 2 the Tl(I) is alternatively attached to a trigonal-bipyramidal Co(CO)3L monoanion by resulting electron-pair donation from the cobalt to the Tl(I). Of prime importance (vide inf ra) is the observed crystallographic difference in the linkage of the Tl(I) to the analogously deformed octahedral-based Pd9 entity in 2 versus that in 1. Table 1 discloses that the Tl−Pd(A) and Tl−Pd(C) mean distances for two independent molecules in 2 are 2.77 Å [range 2.747(1)−2.798(1) Å] and 3.31 Å [range 3.197(1)−3.433(1) Å], respectively. Hence, the Tl(I) atom in 2 is predominantly connected to the three inner octahedral Pd(A) atoms and much less (i.e., as previously stated,1c namely, weak secondary bonding or nonbonding) to the outer three wingtip Pd(C) atoms. A comparison of these Tl−Pd connectivities in 2 with corresponding ones in 1 shows that, whereas the mean for the inner Tl−Pd(A) distances decreases by 0.06 Å from 2.83 Å in 1 to 2.77 Å in 2, the mean for the outer Tl−Pd(C) distances increases by 0.16 Å from 3.15 Å in 1 to 3.31 Å in 2. This overall connectivity change in Tl−Pd coordination may be designated as a (crown-like)Pd6 sextuple (μ3/3-Tl) coordination mode in 1 that is intermediate between the essentially μ3-Tl coordination mode found in 2 and the robust Pd6-crown-stabilized mode that was also previously found for each equivalent Tl(I) in the [Pd6(μ6-Tl)]2(CO)9(PEt3)9]2+ dication (3).1a The pseudo-C3h Tl2Pd12 core in 3 may be described as edge fusions of three Pd5 trigonal bipyramids to a central Tl2Pd3 trigonal bipyramid, such that each Tl(I) is also connected in a similar geometric crownlike Pd6 coordination mode to the three central triangular Pd(A) atoms and three equatorial Pd(C) atoms of the three fused Pd5 trigonal bipyramids (Figure 2). The nearly equivalent mean distances of 2.89 and 2.92 Å for the three Tl−Pd(A) and three Tl−Pd(C) connectivities, respectively, to each naked, capping Tl(I) are consistent with the coordination mode of each Tl(I) in 3 being designated as μ6-Tl. Figure 3 displays the resulting variations in the three different coordination modes of Tl(I) in 1, 2, and 3. This figure thereby emphasizes the presumed stronger relative attachment of Tl(I) to the palladium core upon going from (a) 2 (μ3-Tl) to (b) 1 (μ3/3-Tl) to (c) 3 (μ6-Tl). This relative static structure/bonding trend in the crystalline state for these three Tl−Pd clusters may be correlated with the observed changes in dynamic solution behavior based upon NMR measurements (vide inf ra). Electron-Counting Analysis of 1 and 2 with Bonding Implications Involving the Nucleophilic Electron-Pair Donation of Tl(I). The geometrically analogous structures of 1 and 2, whose metal polyhedra both conform to a distorted Pd(oct)6 octahedron, give rise to the same number of CVEs: namely, 6 × 10 [Pd(A) + Pd(B)] + 3 × 4 [Pd(μ2-CO)2PR3] + 1 × 2 [Tl(I)] + 3 × 2 [(μ3-CO)] + 3 × 2 [PR3] = 86. Since this number is in exact agreement with 86 CVEs predicted by the polyhedral skeletal electron-pair Wade−Mingos model,8 which holds for the vast majority of octahedral-based metal clusters, it suggests that the so-called “inert” electron pair of Tl(I) provides an “energy-stabilizing” influence in both 1 and 2 in occupying one of the 43 CV MOs instead of an additional octahedral antibonding MO.8c Noteworthy is that a previous comparative analysis of the isostructural Tl(I)-(M3)2 sandwiches [(μ6-Tl)M6(μ2-CO)6(PEt3)6]+ (M = Pd, Pt), with the

extensively disordered, it was not included in the detailed analysis of the molecular parameters presented in Tables 1 and 2. The three independently determined nondisordered structures of 1 from the solvated crystals of 1A and 1B possess essentially identical geometries (other than three different orientations of the Tl(acac) plane relative to the octahedralbased Pd9Tl geometry). The TlPd9 core (Figure 1) of 1, which ideally conforms to C3v symmetry, may be viewed as a markedly deformed Pd(A)3Pd(B)3 octahedron (oct) with the three Pd(A) atoms of its Pd(A)3 face connected both to three edge-bridging Pd(C) atoms and to the symmetrically capping Tl atom. Each of these wingtip Pd(C) atoms has a trigonal-planar ligand arrangement, Pd(μ2-CO)2(PPh3), which is common for nanosized CO/PR3ligated palladium clusters. The remaining three PPh3 ligands are coordinated to the octahedral Pd(B) atoms; three triply bridging CO ligands asymmetrically cap the lower nonadjacent Pd(A)Pd(B)2 faces. The resulting Tl−Pd connectivities consist of three inner, shorter Tl−Pd(A) octahedral connectivities of mean distance 2.83 Å [range 2.798(1)−2.877(1) Å] and three outer, longer Tl−Pd(C) wingtip connectivities of mean distance 3.15 Å [range 2.973(1)−3.289(1) Å]. Orientations of the phosphine phenyl rings in both crystal structures 1A and 1B do not indicate any unusual interactions with Tl(I). b. Tl(acac) Fragment. The Tl(I) is equivalently attached to both chelating O atoms of a bidentate acetylacetonate (acac) monoanion. As evidenced from Table 2, the geometries of the pseudo-C2v planar Tl(acac) fragments in all three analyzed molecular structures (i.e., two molecules of 1 in 1A and one molecule in 1B) are essentially the same. However, the orientations of the planar Tl(acac) fragments relative to the octahedral-based Pd9 fragment are markedly different (see Figure 1c,d). The planar Tl(acac) fragment in the molecule of 1 (per se) is ideally presumed to be perpendicular to the octahedral Pd(A)3 face such that its pseudo-C2 axis defined by the vector through Tl and the γ-carbon atom of the acac ligand would be coincident with the pseudo-C3 axis that is perpendicular to the octahedral Pd(A)3 face of the TlPd9 core of 1. The observed angle between these two vectors is 6.8° and 11.4° for the two independent molecules of 1A and 6.2° for the independent molecule of 1B. The corresponding observed interplanar angles between the O2Tl plane and the Pd(A)3 plane (ideally 90°) are 89.9°, 81.9°, and 83.2°. These different orientations of the Tl(acac) fragment may be attributed to its susceptibility to a wobbling-like motion (vide inf ra) in combination with anisotropic crystal-packing effects induced by variable amounts of cocrystallized solvent molecules. The importance of the first factor is consistent with the fact that we did not observe substantial intermolecular interactions in the crystal structures of 1A and 1B. (See Supporting Information.) The short mean Tl−O distances, 2.33−2.36 Å, in 1 indicate significant covalent character of the Tl−O bonding (vide inf ra). These Tl−O distances are ca. 0.1−0.6 Å shorter5 than those found in other compounds having Tl(acac) fragments,2 which include (a) crystalline Tl(acac) with analogous Tl−O chelating distances of 2.52 and 2.53 Å;6a (b) [AuTl2(acac)(C6Cl5)2] and [AuTl3(acac)2(C6F5)2], in which the acac ligands are nonchelating but bridge neighboring Tl atoms with Tl−O connectivities in a range of 2.65−2.82 Å in the first compound and 2.58−2.89 Å in the second one;6a and (c) (CH3)2Tl(acac), for which both Tl(III)−O distances are the same, 2.45(2) Å.6b 2882

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Organometallics

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coupling, which involves broadened doublets with a 4JH−Tl of 5.4−9.6 Hz for the six methyl protons and 4JH−Tl of 31.8−33.6 Hz for the methine proton), definitely indicates the existence of considerable covalent Tl−O interactions between Tl(I) and the acac ligand in solutions of 1.10 This conclusion is consistent with the previously mentioned (0.16−0.20 Å) shorter Tl−O distances in 1 versus those in crystalline Tl(acac).6a Not surprisingly, the acquired 1H NMR spectrum of the solution of Tl(acac) was similar to that of the enol tautomer of Hacac. It showed no H−Tl coupling (singlets at 5.18 and 1.90 ppm), pointing to the ionicity of the Tl bonding,10 which results in a fast exchange of Tl(I) between two oxygen atoms. Temperature-dependent 31 P{ 1 H} and 13 C{ 1H} NMR solution spectra of the closest analogue of 1, Pd9[TlCo(CO)3(PEt3)](CO)9(PEt3)6 (2), provide strong evidence that 2 undergoes completely different dynamic behavior involving rapid reversible dissociation/association of the Tl moiety, whose equilibrium position is strongly shifted to the left-hand side in eq 4:1c

Figure 2. Tl2Pd12 framework of known [Tl2Pd12(CO)9(PEt3)9]2+ dication ([PF6]− salt). This Tl2Pd12 core ideally conforms to C3h symmetry with the 3-fold axis passing through both Tl(I) atoms and the horizontal σh mirror plane passing through both the inner three Pd(A) atoms and outermost three Pd atoms. Its overall geometry may be viewed as edge fusions of three Pd5 trigonal bipyramids to a central Tl2Pd(A)3 trigonal bipyramid. Its chemical formulation may be described as a condensation product of three partially ligated butterfly Pd4(μ2-CO)2(μ3-CO)(PEt3)3 fragments that are stabilized by two capping naked Tl(I) cations. The essentially equivalent six Tl−Pd(A) and six Tl−Pd(C) distances of 2.91 Å (av) are consistent with the crown-like Pd6 coordination mode of three Pd(A) and three Pd(C) atoms linked to each Tl(I), being designated as μ6-Tl.

Pd 9[TlCo(CO)3 L](CO)9 L6 ⥃ Pd 9(CO)9 L6 + TlCo (CO)3 L,

L = PEt3

(4)

In 1, on the other hand, dissociation either of the Tl(acac) fragment from the Pd9 core or of the acac ligand from the Tl(I) must be ruled out, to the extent that any exchange that may be occurring is at a rate less than the smaller observed coupling constant of 5.4−9.6 Hz. In contrast to 2, cooling of a solution of 1 (THF-d8) to −80 °C did not affect the breadth and coupling constants of the signals in both 1H and 31P{1H} NMR spectra. Instead, a rapid wobbling-like motion of the planar C2v Tl(acac) fragment about the pseudo-C3 axis is conjectured to be occurring in 1. (See caption of Figure 1c,d concerning our postulation of a wobbling-like motion.) This complex motion may also involve movements of the Tl atom per se off the pseudo-C3 axis, although with less amplitude (relative to acac wobbling), because of its much shorter radial position. Nevertheless, energy barriers for non-bond-breaking motions are much smaller than those required for dissociation of either the entire Tl(acac) moiety or the acac ligand. Experimentally, these suggestions are supported by (a) the broadened doublets arising from H−Tl coupling, which are essentially unaffected by

Au(I)-(Pt3)2 sandwich, [(μ6-Au)Pt6(μ2-CO)6(PPh3)6]+, provided compelling evidence that the 6s2 electron pair on the Tl(I) exerts an overall “destabilizing” influence relative to the 6s05d10 Au(I). a. Comparative Analysis of Solution Behavior of 1 with Known Tl−Pd Clusters and Relevant Tl−Pt Clusters. Comparison with Pd 9 [TlCo(CO) 3 (PEt 3 )](CO)9(PEt3)6 (2). 1H NMR spectra of solutions of 1 quite unexpectedly showed a H−Tl coupling arising from Tl(acac) that was not observed earlier for other crystallographically characterized compounds with a Tl(acac) entity.6a,9 This

Figure 3. Nearest metal coordination sphere of Tl(I) in (a) Pd9[μ3-TlCo(CO)3PEt3](CO)9(PEt3)6 (2, first independent molecule); (b) Pd9[μ3/3Tl(acac)](CO)9(PPh3)6 (1, first molecule); and (c) one of two equivalent Tl(I) atoms in the [Pd12Tl2(CO)9(PEt3)9]2+ dication (3). These indicated variations in metal coordination sphere emphasize an increasing stabilization of Tl(I) by the Pd6-crown fragment (left to right) in 2 (μ3-Tl), 1 (μ3/3Tl), and 3 (μ6-Tl). Mean interplanar Pd(A)2Pd(C)−Pd(A)3 angles are 161.6° in 2; 153.7° in 1; and 141.5° in 3. Perpendicular distances from Tl to the Pd(C)3 plane are 1.42 Å in 2; 1.23 Å in 1; and 0.86 Å in 3. Longer Tl−Pd(C) bonding connectivities in 1 are indicated by dashed lines. 2883

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N2, unless otherwise stated. 31P{1H} and 1H NMR spectra were obtained on a Bruker AC-300 or INOVA-500 spectrometer and referenced, respectively, to 85% H3PO4 in D2O as an external standard and to residual protons of the internal deuterated solvents or TMS (at 300 MHz). At 500 MHz, the “unified scale” method of referencing was used16 for referencing 31P from the 1H TMS reference. IR spectra were recorded on a Bruker Tensor 27 FT-IR spectrometer or a Mattson Polaris FT-IR spectrometer with samples suspended in Nujol or obtained in a solution. X-ray diffraction data collections were performed on Bruker area-detector diffractometers. Synthesis of Pd9[Tl(acac)](CO)9(PPh3)6 (1) in the Presence of Na2CO3: Isolation of Pd9[Tl(acac)](CO)9(PPh3)6·1.5Hacac·0.5THF (1A). A solution of Pd10(CO)12(PPh3)6 (0.150 g, 0.05 mmol) and PPh3 (0.053 g, 0.20 mmol) in THF (8 mL) was stirred under CO for 15 min. The CO atmosphere was replaced by N2, and acetylacetone (2 mL, 19.6 mmol) and Na2CO3 (0.2 g, 1.9 mmol) were added. After being stirred for 10 min, a solution of TlPF6 (58 mg, 0.166 mmol) in 4 mL of THF was added dropwise followed by continuous stirring for one day. The solution was evaporated to a volume of ∼3 mL, filtered, and kept under vapors of i-Pr2O. Black-red crystals of 1 and colorless ones of Na2CO3 were separated by decantation and, after being washed with H2O and MeOH and dried under vacuum, afforded 0.045 g (26%, based on Pd) of crystals of Pd9[Tl(acac)](CO)9(PPh3)6 (1). From this batch a single crystal of Pd9[Tl(acac)](CO)9(PPh3)6·1.5Hacac·0.5THF (1A) of size 0.41 × 0.24 × 0.15 mm3 was selected for X-ray data collection. Although this particular crystal possessed 1.5 solvent molecules of acetylacetone per cluster, the bulk crystals did not contain a detectable amount of Hacac, as evidenced by IR spectroscopy. In fact, crystallization of 1 occurs with variable amounts of cocrystallized Hacac and THF solvent molecules (see also the Supporting Information). IR spectrum of 1 (in Nujol) ν(CO): 1927 vw sh, 1903 sh, 1896 s, 1874 w sh, 1863 m-w, 1854 w sh, 1832 m, 1803 m-s, 1799 m-s (these latter two peaks overlapped); bands of acac monoanion: 1579 s br, 1513 m, cm−1. Synthesis of Pd9[Tl(acac)](CO)9(PPh3)6 (1) in the Presence of NEt3: Isolation of Pd9[Tl(acac)](CO)9(PPh3)6·0.3THF (1B). A mixture of Pd10(CO)12(PPh3)6 (0.200 g, 0.067 mmol) and PPh3 (0.013 g, 0.050 mmol) in THF (14 mL) was stirred under CO for 15 min. The atmosphere was changed to N2, and a mixture of acetylacetone (2.0 mL, 19.6 mmol) and NEt3 (2.0 mL, 14.3 mmol) was added followed by dropwise addition of a solution of TlPF6 (29 mg, 0.083 mmol) in 4 mL of THF. The resulting dichroic brownishgreen (reflection)/crimson (transmission) solution was stirred under N2 flow (ca. 1.5 h) until its volume decreased to ca. 3 mL. The solution was filtered and after one day of crystallization in the presence of vapors of i-Pr2O gave rise to dark red crystals, which were washed with i-Pr2O and MeOH and dried under vacuum. This procedure afforded 0.185 g (80%, based on Pd) of Pd9[Tl(acac)](CO)9(PPh3)6 (1). The same procedure using 2-fold amounts of all reagents afforded 0.435 g (94%) of crystalline 1. One of the crystals of size 0.163 × 0.133 × 0.041 mm3 selected from the batch of 0.185 g, Pd9[Tl(acac)](CO)9(PPh3)6·0.3THF (1B), was used for the X-ray data collection. The crystallographically determined number of THF solvent molecules per molecule of 1 is in excellent agreement with that (viz., 0.28) estimated from 1H NMR. Nevertheless, the number of THF molecules is variable and could be as low as 0.1 per 1 (by 1H NMR estimations of other batches). IR spectra of 1 (in Nujol) ν(CO): 1933 w-m, 1930 sh, 1904 vs, 1894 sh, 1880 w sh, 1874 w sh, 1854 w-m sh, 1841 w-m, 1832 w-m, 1806 m sh, 1796 m-s; bands of acac−: 1578 s-m, 1571 sh, 1559 sh, 1541 w, 1512 m, 1479 m, 1434 s, cm−1; in CHCl3, ν(CO): 1930 vw sh, 1902 vs, 1880 vw sh, 1840 w-m, 1809 m; bands of acac−: 1580 m, 1574 sh, 1567 sh, 1512 w, 1479 w, 1435 m-s, cm−1. 31P{1H} NMR (121.395 MHz) in CDCl3: δ1 = 17.17 ppm (d, 3P, P(C), 2,3JP(C)‑Tl = 333 Hz), δ2 = 14.88 ppm (d, 3P, P(B), 3JP(B)‑Tl = 148 Hz), with halfwidth at half-height of 11 Hz and the intensity ratios of δ1/δ2 = 2.01/ 2.04 (theoretical is 2.00/2.00). 1H NMR (299.874 MHz) in CDCl3: δ1 = 7.3−6.8 ppm (m, 90H, C6H5, overlapped with the singlet, 7.24 ppm, from the residual protons of the deuterated chloroform), δ2 = 4.16 ppm (d, 1H, CH, acac, 4JH−Tl = 32.7 Hz), δ3 = 3.75 ppm (m, 4H,

temperature changes, and to some extent by (b) geometrical diversity in the Tl(I) chelation of the acac fragment, as suggested from the crystal structures of 1A and 1B. In comparison with 2, it is unclear whether the indicated stronger Tl···Pd(C) interactions in 1 (per se) or the combined bonding effect also involving both the replacements of PEt3 ligands in 2 by PPh3 ones in 1 and the terminal anionic Co(CO)3L ligand in 2 by a Tl(I)-chelating acac monoanion in 1 is responsible for the presumed overall greater strength of the Tl(I)−Pd(0) bonding interactions in 1. b. Comparison with [Pd12Tl2(CO)9(PEt3)9]2+ Dication (3). 31 1 P{ H} NMR spectra of 1 are similar to those of 31a in terms of both the signal breadth and the essentially unchanging character of these signals upon a temperature decrease. This similarity indicates the possibility of a dynamic solution structure involving small rapid displacements of each Tl(I) atom off the pseudo-C3 axis in 3 in a manner analogous to that in 1. However, the absence of a H-marker in the coordination hemisphere of Tl(I) in 3 (in contrast to that in 1) does not allow any confirmation of this possible motion by 1H NMR. c. Comparison with Sandwich [Pd6Tl(CO)6(PEt3)6]+ Monocation. The strong indication by 1H NMR data of signif icant covalent character in the linkage of the acac ligand to Tl(I), which is attached to the template-functioning Pd9 core in 1, is also of particular interest for the following reason. The normal tendency of 6s2 Tl(I) in many compounds to have at least half of its coordination sphere “seemingly” unoccupied is well known11 and is commonly considered to be a consequence of the “inert-pair effect”.12 A striking example of this tendency is the spontaneous conversion of the M3TlM3 sandwich [Pd6Tl(CO)6(PEt3)6]+ monocation, in which both of its coordination hemispheres are symmetrically occupied by two unconnected triangular Pd3(μ2-CO)3(PEt3)3 units, into the above-mentioned [Pd12Tl2(CO)9(PEt3)9]2+ cluster dication (3). This conversion is achieved by relocation of all Pd atoms into one coordination hemisphere for each Tl(I), leaving the second one vacant, but yet giving rise to a robust (μ6-Tl)Pd6-crown stabilization (Figure 3c).1b d. Comparison with Relevant Tl(I)−Pt(0) Clusters. In general, much stronger Pt−Pt bonds than the Pd−Pd ones4 seemingly do not affect the dynamic behavior of the Pt(0)− Tl(I) clusters in solution, which is quite sensitive to both Pt and Tl ligand environments.13,9,1b For a number of pseudotetrahedral Pt3Tl clusters, [Pt3{μ3-Tl(diketonate)(H2O)}(μ3-CO)(μdppm)3]2+ (including Tl(acac); dppm = PPh2CH2PPh2, all isolated as [PF6]− salts), in addition to the chemically observed reversible attachment of the Tl(diketonate) fragments to the Pt3 triangles, intermolecular exchange of the diketonate and H2O ligands was suggested on the basis of the absence of H−Tl coupling in 1H NMR spectra.9 In the related tetrahedral Pt3Re cluster, [Pt3{μ3-Tl(acac)}(ReO3)(μ-dppm)3]+, which was characterized spectroscopically, the broadened signals of the acac protons are consistent with the proposed easy rotation (in solution) of the Tl(acac) unit about the assumed 3-fold Pt3 axis in accordance with a singlet dppm resonance (a 1:1 doublet due to 2JP−Tl coupling).13b

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EXPERIMENTAL SECTION

General Remarks and Instrumentation. Pd10(CO)12(PPh3)6 was prepared by a literature method.14 Tl(acac) was obtained from TlNO3 on the basis of the procedure used for preparation of thallium(I) benzoylacetonate.15 All other reagents were purchased and used without further purification. All operations were performed under 2884

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−CH2-O-CH2−), δ4 = 1.85 ppm (m, 4H, C-CH2-CH2-C), δ5 = 0.85 ppm (d, 6H, CH3, acac, 4JH−Tl = 9.0 Hz), with intensity ratios of δ2/δ3/ δ4/δ5 = 0.97/1.18/1.04/6.00 and of (δ3 + δ4)/δ5 = 2.22/6.00 that corresponds to 0.28 THF per one acac− ligand or per 1. The theoretical ratio for δ2/δ5 is 1.00/6.00. Attempts to obtain 203,205Tl NMR spectra of 1 were unsuccessful. 1 H NMR (299.874 MHz) of Tl(acac) in CDCl3: δ1 = 5.18 ppm (s, 1H, CH), δ2 = 1.90 ppm (s, 6H, CH3), with intensity ratio of δ1/δ2 = 0.98/6.00 (theoretical is 1.00/6.00). Synthesis of Pd9[Tl(acac)](CO)9(PPh3)6 (1) in Neutral Media by Direct Use of Tl(acac). A mixture of Pd10(CO)12(PPh3)6 (0.36 g, 0.121 mmol), PPh3 (0.024 g, 0.092 mmol), and Tl(acac) (0.045 g, 0.148 mmol) in THF (16 mL) was stirred for 0.5 h. The solvent was evaporated to ca. 8 mL; after addition of heptane (ca. 5 mL) a dark red-brown crystalline precipitate was isolated and identified spectroscopically as Pd9[Tl(acac)](CO)9(PPh3)6 (1), 0.390 g (94%, based on Pd). X-ray Crystallographic Determinations of Pd9[Tl(acac)](CO) 9 (PPh 3 ) 6 ·1.5Hacac·0.5THF (1A) and Pd 9 [Tl(acac)](CO)9(PPh3)6·0.3THF (1B). General Remarks. X-ray data for crystals 1A and 1B were collected at 100(2) K with a Bruker SMART CCD1000 area detector diffractometer (Mo Kα radiation, λ = 0.71073 Å) and with a Bruker Apex2 diffractometer (Cu Kα radiation, λ = 1.54178 Å), respectively. Reflections from crystal 1A were empirically corrected for absorption (SADABS),17 whereas reflections from crystal 1B were analytically corrected.18 The crystal structures were obtained by use of direct methods followed by difference Fourier maps. Least-squares refinements (based on F2) were carried out with SHELXTL.17,18 Both crystal structures 1A and 1B contain two independent molecules of Pd9[Tl(acac)](CO)9(PPh3)6 (1). The second molecule of 1 in the crystal structure of 1B has a disordered fragment consisting of the wingtip Pd(13) atom with its P(C6H5)3 ligand (P7 atom) with occupancies of 0.90/0.10, one disordered CH3 group of the acac ligand (0.55/0.45), and three disordered C6H5 groups connected to P8 (one, 0.68/0.32) and P11 (two, 0.62/0.38 and 0.70/0.30). In the minor component of the disordered Pd−P(C6H5)3 fragment (occupancy 0.1) only Pd and P atoms were refined, whereas the atoms of the phenyl substituents were not localized due to insufficient electron density. All non-hydrogen atoms with the exception of the C atoms of the disordered CH3 and C6H5 groups and the C, O atoms of THF solvent molecule (all are in 1B) were refined anisotropically. The hydrogen atoms were generated geometrically and refined as riding atoms with corresponding default C−H distances. In the crystal structure of 1B aromatic C atoms were fit to a regular hexagon with C−C distances of 1.390 Å. In the crystal structures of 1A and 1B the observed highest residual positive peaks of 6.91 and 3.52 e·A−3 were found in the vicinities of Tl and Pd atoms, respectively, and attributed to noise. Structural CIF/PLATON tests performed by the http://journals.iucr. org/services/cif/checking/checkform.html were in accordance with the crystal structure determinations of 1A and 1B. CCDC reference numbers are 853917 (1A) and 853918 (1B). Pd9[Tl(acac)](CO)9(PPh3)6·1.5Hacac·0.5THF (1A): C131.5H113O14.5P6Pd9Tl; M = 3273.01 g·mol−1; triclinic; P1̅; Z = 4; a = 17.5842(8) Å, b = 26.4171(13) Å, c = 26.6018(13) Å, α = 88.810 (1)°, β = 88.724(1)°, γ = 80.675(1)°; V = 12189.0(10) Å3; d(calc) = 1.784 Mg/m3; F(000) = 6412. Reflections (113 530) obtained over 1.54° ≤ 2θ ≤ 56.62° (98.9% completeness to 2θ = 50.00°); max./min. transmission coefficients, 0.6830/0.3983; μ(Mo Kα) = 2.752 mm−1. Full-matrix least-squares refinement on 58 013 independent merged (R(int) = 0.0393) reflections (2917 parameters, 85 restraints) converged at wR2(F2) = 0.1771 for all data; R1(F) = 0.0644 for I > 2σ(I); GOF (on F2) = 1.083; max./min. residual electron density, 6.914 and −2.752 e·A−3. Pd9[Tl(acac)](CO)9(PPh3)6·0.3THF (1B): C123.20H99.40O11.30P6Pd9Tl; 3108.42 g·mol−1; triclinic; P1̅; Z = 4; a = 16.6815(5) Å, b = 23.9936(7) Å, c = 30.3230(9) Å, α = 107.682(2)°, β = 92.176(2)°, γ = 99.572(2)°; V = 11351.7(6) Å3; d(calc) = 1.819 Mg/m3; F(000) = 6056. Reflections (180 753) obtained over 3.08° ≤ 2θ ≤ 144.54° (96.8% completeness to 2θ = 134.00°); max./min. transmission coefficients, 0.5754/0.1914; μ(Cu Kα) = 15.154 mm−1.

Full-matrix least-squares refinement on 42 046 independent merged (R(int) = 0.0489) reflections (2127 parameters, 159 restraints) converged at wR2(F2) = 0.1506 for all data; R1(F) = 0.0448 for I > 2σ(I); GOF (on F2) = 1.078; max./min. residual electron density, 3.521 and −2.029 e·A−3.

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ASSOCIATED CONTENT

S Supporting Information *

Recorded IR, 31P{1H}, 1H NMR, and SEM spectra and X-ray crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation, UOP LLC (Des Plaines, IL), and the University of Wisconsin−Madison. The SMART 1000 CCD X-ray area detector system was purchased, in part, from NSF grant CHE9310428. The Varian Inova-500 NMR spectrometer was purchased, in part, from NSF grant CHE-9629688; and the Bruker AC-300 NMR spectrometer, in part, by funds from NSF CHE-9208963 and NIH SIO RR 08389-01. We thank Dr. Ilia Guzei (UW−Madison) for crystallographic assistance and Dr. Richard Noll (Materials Science Center, UW−Madison) for performing the X-ray Tl/Pd microanalyses of 1.

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DEDICATION Dedicated to the late Professor F. Gordon A. Stone (1925− 2011) on the occasion of his retirement as the Distinguished Robert A. Welch Professor of Chemistry at Baylor University in 2010. His illustrious career over this last half-century places him among the giants who have impacted the advancement of modern synthetic inorganic/organometallic chemistry.

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REFERENCES

(1) (a) Ivanov, S. A.; Nichiporuk, R. V.; Mednikov, E. G.; Dahl, L. F. J. Chem. Soc., Dalton Trans. 2002, 4116−4127. (b) Mednikov, E. G.; Dahl, L. F. Dalton Trans. 2003, 3117−3125. (c) Mednikov, E. G.; Fry, C. G.; Dahl, L. F. Angew. Chem., Int. Ed. 2005, 44, 786−790. (2) The Cambridge Crystallographic Data Centre: www.ccdc.cam.ac. uk. (3) (a) Labios, L. A.; Millard, M. D.; Rheingold, A. L.; Figueroa, J. S. J. Am. Chem. Soc. 2009, 131, 11318−11319. (b) Balch, A. L.; Davis, B. J.; Fung, E. Y.; Olmstead, M. M. Inorg. Chim. Acta 1993, 212, 149− 156. (c) Catalano, V. J.; Bennett, B. L.; Yson, R. L.; Noll, B. C. J. Am. Chem. Soc. 2000, 122, 10056−10062. (d) Catalano, V. J.; Malwitz, M. A. J. Am. Chem. Soc. 2004, 126, 6560−6561. (e) Lanci, M. P.; Remy, M. S.; Kaminsky, W.; Mayer, J. M.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 15618−15620. (4) (a) Mednikov, E. G.; Dahl, L. F. Phil. Trans. R. Soc. A 2010, 368, 1301−1332 , and references therein. (b) Mednikov, E. G.; Dahl, L. F. J. Chem. Educ. 2009, 86, 1135. (5) (a) The exception is [{CpMo(CO)3}2Tl(acac)],5b where both Tl−O distances, 2.33 Å, are similar to that in 1. (b) Hughes, R. P.; Rheingold, A. L.; Allen, M. B. Private communication, 1996, CCDC refcode NACQOX. (6) (a) Fernandez, E. J.; Laguna, A.; Lopez-de-Luzuriaga, J. M.; Monge, M.; Montiel, M.; Olmos, M. E.; Perez, J. Organometallics 2004, 23, 774−782. (b) Chow, Y. M.; Britton, D. Acta Crystallogr. 1975, B31, 1929−1934. 2885

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(7) (a) The Pd9(CO)9P6 fragment in 1 is also geometrically analogous to that recently reported in a crystal structure determination of the PPh3-ligated [Pd9Au(CO)9(PPh3)6]+ monocation isolated as the [PF6]− salt.7b This presumed compound was isolated in very low yield from the reaction of Pd3(CO)3(PBut3)3 with Au(PPh3)Cl in the presence of TlPF6. However (as stated by authors), “its synthesis could not be reproduced, and as Au and Tl are non-distinguishable by the Xray analysis, the actual nature of the heteroatom could not be determined”. Hence, they stated, “It is therefore not possible to exclude the possibility that Tl could be incorporated in the structure”. This conclusion was based upon their literature reference to Pd12Tl2(CO)9(PEt3)9][PF6]21a but not to that of 2. We just obtained [Pd9Tl(CO)9(PPh3)6][PF6] (yield, 70%), for which the crystal structure lattice parameters and the refined crystal structure are analogous to those reported in 7b. Hence, we currently believe that their isolated compound indeed is [Pd9Tl(CO)9(PPh3)6][PF6]. Noteworthy is that both the cation and [PF6]− anion lie on a crystallographic 3-fold axis with the [PF6]− anion highly disordered. (b) Willocq, C.; Hermans, S.; Devillers, M.; Tinant, B. Z. Kristallogr. 2008, 223, 495−497. (8) (a) Mingos, D. M. P. Acc. Chem. Res. 1984, 17, 311−319. (b) Wade, K. Adv. Inorg. Radiochem 1976, 18, 1−66. (c) Lauher, J. W. J. Am. Chem. Soc. 1978, 100, 5305−5315. (9) Stadnichenko, R.; Sterenberg, B. T.; Bradford, A. M.; Jennings, M. C.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 2002, 1212−1216. (10) Janiak, C. Coord. Chem. Rev. 1997, 163, 107−216. (11) (a) Kristiansson, O. Eur. J. Inorg. Chem. 2002, 2355−2361. (b) Wiesbrock, F.; Schmidbaur, H. J. Am. Chem. Soc. 2003, 125, 3622− 3630. (c) Huang, S.-H.; Wang, R.-J.; Mak, T. C. W. J. Chem. Soc., Dalton Trans. 1991, 1379−1381. (d) Akhbari, K.; Morsali, A. Coord. Chem. Rev. 2010, 254, 1977−2006. (12) (a) Pitzer, K. S. Acc. Chem. Res. 1979, 12, 271−276. (b) Pyykkö, P.; Desclaux, J.-P. Acc. Chem. Res. 1979, 12, 276−281. (13) (a) Ezomo, O. J.; Mingos, D. M. P.; Williams, I. D. J. Chem. Soc., Chem. Commun. 1987, 924−925. (b) Hao, L.; Xiao, J.; Vittal, J. J.; Puddephatt, R. J.; Manojlović-Muir, L.; Muir, K. W.; Torabi, A. A. Inorg. Chem. 1996, 35, 658−666. (c) de Silva, N.; Fry, C. G.; Dahl, L. F. Dalton Trans. 2006, 1051−1059. (14) Mednikov, E. G.; Eremenko, N. K.; Gubin, S. P. Koordin. Khim. (Sov. J. Coord. Chem.) 1984, 10, 711−714. (15) Nelson, W. H.; Randall, W. J.; Martin, D. F. Inorg. Synth. 1967, 9, 52−55. (16) Harris, R. K.; Becker, E. D.; Cabral de Menezes, S. M.; Goodfellow, R.; Granger, P. Pure Appl. Chem. 2001, 73, 1795. (17) Sheldrick, G. M. SHELXTL (version 6.10) Program Library; Bruker Analytical X-Ray Systems: Madison, WI, 2000. (18) Apex2 v2010.7-0; Bruker-AXS Inc.: Madison, WI, 2009.

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NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper published on March 5, 2012, the formula for one of the structures shown in the abstract and table of contents graphics was incorrect. In the version of the paper that appears as of March 7, 2012, the formula is correct.

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