Construction of a Planar Tetrapalladium Cluster by the Reaction of Palladium ( 0 ) Bis ( isocyanide ) with Cyclic Tetrasilane

The planar tetrapalladium cluster Pd4{Si(Pr)2}3(CNBu)4 (4) was synthesised in 86% isolated yield by the reaction of palladium(0) bis(isocyanide) Pd(CNBu)2 with octaisopropylcyclotetrasilane (3). In the course of this reaction, the palladium atoms are clustered via insertion into the Si–Si bonds of 3, followed by extrusion of one SiPr2 moiety and reorganisation to afford 4 with a 54-electron configuration. The CNtBu ligand in 4 was found to be easily replaced by N-heterocyclic carbene (Pr2IM) to afford the more coordinatively unsaturated cluster Pd4{Si(Pr)2}3(Pr2IM)3 (5) having the planar Pd4Si3 core. On the other hand, the replacement of CNtBu with a sterically compact ligand trimethylolpropane phosphite {P(OCH2)3CEt} led to a planar tripalladium cluster Pd3{Si(Pr)2}3{P(OCH2)3CEt}3 (6) and Pd{P(OCH2)3CEt}4 in 1:1 molar ratio as products.


Introduction
Transition metal clusters have attracted much attention because of their unique chemical properties.These clusters have been extensively studied in homogeneous catalysis, in which the substrate can be cooperatively activated by dual metal components in the cluster [1][2][3].In many cases, the clusters are also used as structural models of the active sites in heterogeneous catalysts.Detailed spectroscopic analysis of the substrates coordinated to the cluster framework has provided unique insight into the function of the active surface in, for example, the chemisorption process [4,5].Moreover, some of the clusters provide structural and/or functional mimics of the active sites in enzymes [6][7][8][9].A typical way to prepare the clusters is the self-assembly of metals and bridging ligands, by simply mixing the appropriate metal precursors in proper molar ratios under suitable reaction conditions.However, the self-assembly process generally forms a mixture of clusters of different nuclearities.Thus, a separation step is required to isolate the desired clusters, thereby lowering the yield of the product.In addition, the molecular structure of the formed clusters is not very predictable; in other words, this strategy is unsuitable for custom-designed clusters.
We are interested in producing more 2D sheet-like clusters based on the template synthesis approach, focusing on the use of cyclic organopolysilanes as the template molecules.In our previous study, we found that a ladder polysilane, decaisopropylbicyclo[2.2.0]hexasilane (1), acts as a good template molecule for the preparation of large palladium clusters [29].It is known that some transition metal species can be inserted into the Si-Si bonds of organosilanes [30,31], and "Pd(CNR) 2 (CNR = isocyanide)" is one of the representative [32,33].In the reaction of Pd(CN t Bu) 2 with 1, the Pd 11 cluster Pd 11 {Si( i Pr)} 2 {Si( i Pr) 2 } 4 (CN t Bu) 10 (2), having a "folding" nanosheet structure, was confirmed to form in high isolated yield.As shown in Scheme 1, 1 behaves as a template to fix seven palladium species between seven different Si-Si bonds.Four additional palladium species participated to form the Pd 11 cluster 2. In addition, the ligand exchange of CN t Bu in 2 to CN(2,4,6-Me 3 -C 6 H 2 ) triggered the skeletal rearrangement to produce another Pd 11 cluster to have a "folding" nanosheet structure but with a wider dihedral angle.This indicates that the structure of the cluster can be tuned by changing the auxiliary ligand on the metal.Theoretical calculations were also used to elucidate the electronic structure and bonding nature of these clusters [34].
We are interested in producing more 2D sheet-like clusters based on the template synthesis approach, focusing on the use of cyclic organopolysilanes as the template molecules.In our previous study, we found that a ladder polysilane, decaisopropylbicyclo[2.2.0]hexasilane (1), acts as a good template molecule for the preparation of large palladium clusters [29].It is known that some transition metal species can be inserted into the Si-Si bonds of organosilanes [30,31], and "Pd(CNR)2 (CNR = isocyanide)" is one of the representative [32,33].In the reaction of Pd(CN t Bu)2 with 1, the Pd11 cluster Pd11{Si( i Pr)}2{Si( i Pr)2}4(CN t Bu)10 ( 2), having a "folding" nanosheet structure, was confirmed to form in high isolated yield.As shown in Scheme 1, 1 behaves as a template to fix seven palladium species between seven different Si-Si bonds.Four additional palladium species participated to form the Pd11 cluster 2. In addition, the ligand exchange of CN t Bu in 2 to CN(2,4,6-Me3-C6H2) triggered the skeletal rearrangement to produce another Pd11 cluster to have a "folding" nanosheet structure but with a wider dihedral angle.This indicates that the structure of the cluster can be tuned by changing the auxiliary ligand on the metal.Theoretical calculations were also used to elucidate the electronic structure and bonding nature of these clusters [34].Along the same line, in this study we intend to use octaisopropylcyclotetrasilane (3) (Chart 1) having four Si-Si bonds as a template for the clustering of palladium species.As we expected, the reaction of 3 with Pd(CN t Bu)2 effectively formed a planar tetrapalladium (Pd4) cluster framework.Meanwhile, an unexpected outcome was that the reaction was accompanied by extrusion of one Si i Pr2 moiety, leading to the formation of a coordinatively unsaturated cluster Pd4{Si( i Pr)2}3(CN t Bu)4 (4) with 54-electron configuration as a single product.Ligand exchange of CN t Bu in 4 by an Nheterocyclic carbene ( i Pr2IM Me ) proceeded with preserved Pd4Si3 core structure to afford a planar Pd4 cluster Pd4{Si( i Pr)2}3( i Pr2IM Me )3 (5) quantitatively, which has 52 electrons and coordinatively more unsaturated than 4. On the other hand, ligand exchange reaction of 4 with trimethylolpropane phosphite decreased the cluster nuclearity to afford the planar tripalladium cluster Pd3{Si( i Pr)2}3{P(OCH2)3CEt}3 ( 6) and the mononuclear Pd{P(OCH2)3CEt}4 concomitantly.Along the same line, in this study we intend to use octaisopropylcyclotetrasilane (3) (Chart 1) having four Si-Si bonds as a template for the clustering of palladium species.As we expected, the reaction of 3 with Pd(CN t Bu) 2 effectively formed a planar tetrapalladium (Pd 4 ) cluster framework.Meanwhile, an unexpected outcome was that the reaction was accompanied by extrusion of one Si i Pr 2 moiety, leading to the formation of a coordinatively unsaturated cluster Pd 4 {Si( i Pr) 2 } 3 (CN t Bu) 4 (4) with 54-electron configuration as a single product.Ligand exchange of CN t Bu in 4 by an N-heterocyclic carbene ( i Pr 2 IM Me ) proceeded with preserved Pd 4 Si 3 core structure to afford a planar Pd 4 cluster Pd 4 {Si( i Pr) 2 } 3 ( i Pr 2 IM Me ) 3 (5) quantitatively, which has 52 electrons and coordinatively more unsaturated than 4. On the other hand, ligand exchange reaction of 4 with trimethylolpropane phosphite decreased the cluster nuclearity to afford the planar tripalladium cluster Pd 3 {Si( i Pr) 2 } 3 {P(OCH 2 ) 3 CEt} 3 (6) and the mononuclear Pd{P(OCH 2 ) 3 CEt} 4 concomitantly.

Synthesis of Planar Pd4 Cluster 4 by Reaction of Pd(CN t Bu)2 with Cyclotetrasilane 3
As mentioned in the introduction, Pd(CN t Bu)2 is known to show high reactivity toward insertion into the Si-Si bonds of various organosilanes.Indeed, the reaction of Pd(CN t Bu)2 with 3 proceeded smoothly in toluene at 65 °C to afford the planar Pd4 cluster 4 in 86% isolated yield (Scheme 2).By monitoring this reaction by 1 H NMR spectroscopy, we found that 4 was formed as the sole product, since no other silicon-containing by-products were observed.This indicates that all the silylene (Si i Pr2) moieties in 3 were incorporated into 4.In addition, no intermediary palladium species were found in this reaction.It should be noted that Osakada et al. synthesized a structurally similar planar Pd4 cluster having three bridging silylene ligands (Chart 1) by the reaction of dinuclear {Pd(PCy3)}2(μη 2 -SiHPh2)2 and 1,2-bis(dimethylphosphino)ethane (dppm) at 80 °C [35].In that cluster, dppm was used as the auxiliary ligand for the palladium centre on the edge.In contrast, two of the three palladium atoms on the edge of 4 (Pd(1) and Pd(2) in Figure 1) bear only one isocyanide ligand each, giving a more coordinatively unsaturated cluster with 54 cluster valence electrons, compared with that prepared by Osakada et al.The molecular structure of 4 was determined by X-ray diffraction analysis.The ORTEP drawing of 4 is given in Figure 1a, and the side view of its core Pd4Si3 fragment is shown in Figure 1b.The selected bond distances are summarized in Table 1.Three palladium atoms and three silicon atoms derived from the bridging silylene moieties form an almost planar six-membered ring, and the fourth palladium atom (Pd(4)) is located at the centre of this ring.Therefore, the four palladium atoms and three silicon atoms lie on a plane, and deviations of all seven atoms from this plane are within the range of 0.028-0.178Å.The Pd(4)-Pd(1)-C(CN t Bu) and Pd(4)-Pd(2)-C(CN t Bu) axes slightly deviate from linearity (162.37 (14) and 163.14(14)°, respectively), which may originate from the strong transinfluence of the central Pd(4) atom whose formal oxidation state is Pd(0).The sum of the two Pd(cent)-Si-Cipso angles and the Cipso-Si-Cipso angle is close to 360° (355.5°-360.0°).This suggests that the coordination geometry around the Si atoms can be regarded as pseudo-trigonal bipyramidal, and the two Pd(edge) atoms for each Si atom (Pd(1) and Pd(2) for Si(1), Pd(2) and Pd(3) for Si(2), and Chart 1. Structures of octaisopropylcyclotetrasilane 3, palladium clusters having bridging silylene ligands, and N-heterocyclic carbene ligand ( i Pr 2 IM Me ).As mentioned in the introduction, Pd(CN t Bu) 2 is known to show high reactivity toward insertion into the Si-Si bonds of various organosilanes.Indeed, the reaction of Pd(CN t Bu) 2 with 3 proceeded smoothly in toluene at 65 • C to afford the planar Pd 4 cluster 4 in 86% isolated yield (Scheme 2).By monitoring this reaction by 1 H NMR spectroscopy, we found that 4 was formed as the sole product, since no other silicon-containing by-products were observed.This indicates that all the silylene (Si i Pr 2 ) moieties in 3 were incorporated into 4.In addition, no intermediary palladium species were found in this reaction.It should be noted that Osakada et al. synthesized a structurally similar planar Pd 4 cluster having three bridging silylene ligands (Chart 1) by the reaction of dinuclear {Pd(PCy 3 )} 2 (µ-η 2 -SiHPh 2 ) 2 and 1,2-bis(dimethylphosphino)ethane (dppm) at 80 • C [35].In that cluster, dppm was used as the auxiliary ligand for the palladium centre on the edge.In contrast, two of the three palladium atoms on the edge of 4 (Pd(1) and Pd(2) in Figure 1) bear only one isocyanide ligand each, giving a more coordinatively unsaturated cluster with 54 cluster valence electrons, compared with that prepared by Osakada et al.

Synthesis of Planar Pd4 Cluster 4 by Reaction of Pd(CN t Bu)2 with Cyclotetrasilane 3
As mentioned in the introduction, Pd(CN t Bu)2 is known to show high reactivity toward insertion into the Si-Si bonds of various organosilanes.Indeed, the reaction of Pd(CN t Bu)2 with 3 proceeded smoothly in toluene at 65 °C to afford the planar Pd4 cluster 4 in 86% isolated yield (Scheme 2).By monitoring this reaction by 1 H NMR spectroscopy, we found that 4 was formed as the sole product, since no other silicon-containing by-products were observed.This indicates that all the silylene (Si i Pr2) moieties in 3 were incorporated into 4.In addition, no intermediary palladium species were found in this reaction.It should be noted that Osakada et al. synthesized a structurally similar planar Pd4 cluster having three bridging silylene ligands (Chart 1) by the reaction of dinuclear {Pd(PCy3)}2(μη 2 -SiHPh2)2 and 1,2-bis(dimethylphosphino)ethane (dppm) at 80 °C [35].In that cluster, dppm was used as the auxiliary ligand for the palladium centre on the edge.In contrast, two of the three palladium atoms on the edge of 4 (Pd(1) and Pd(2) in Figure 1) bear only one isocyanide ligand each, giving a more coordinatively unsaturated cluster with 54 cluster valence electrons, compared with that prepared by Osakada et al.The molecular structure of 4 was determined by X-ray diffraction analysis.The ORTEP drawing of 4 is given in Figure 1a, and the side view of its core Pd4Si3 fragment is shown in Figure 1b.The selected bond distances are summarized in Table 1.Three palladium atoms and three silicon atoms derived from the bridging silylene moieties form an almost planar six-membered ring, and the fourth palladium atom (Pd(4)) is located at the centre of this ring.Therefore, the four palladium atoms and three silicon atoms lie on a plane, and deviations of all seven atoms from this plane are within the range of 0.028-0.178Å.The Pd(4)-Pd(1)-C(CN t Bu) and Pd(4)-Pd(2)-C(CN t Bu) axes slightly deviate from linearity (162.37 (14) and 163.14( 14)°, respectively), which may originate from the strong transinfluence of the central Pd(4) atom whose formal oxidation state is Pd(0).The sum of the two Pd(cent)-Si-Cipso angles and the Cipso-Si-Cipso angle is close to 360° (355.5°-360.0°).This suggests that the coordination geometry around the Si atoms can be regarded as pseudo-trigonal bipyramidal, and the two Pd(edge) atoms for each Si atom (Pd(1) and Pd(2) for Si(1), Pd(2) and Pd(3) for Si(2), and The molecular structure of 4 was determined by X-ray diffraction analysis.The ORTEP drawing of 4 is given in Figure 1a, and the side view of its core Pd 4 Si 3 fragment is shown in Figure 1b.The selected bond distances are summarized in Table 1.Three palladium atoms and three silicon atoms derived from the bridging silylene moieties form an almost planar six-membered ring, and the fourth palladium atom (Pd(4)) is located at the centre of this ring.Therefore, the four palladium atoms and three silicon atoms lie on a plane, and deviations of all seven atoms from this plane are within the range of 0.028-0.178Å.The Pd(4)-Pd(1)-C(CN t Bu) and Pd(4)-Pd(2)-C(CN t Bu) axes slightly deviate from linearity (162.37 (14) and 163.14(14) • , respectively), which may originate from the strong trans-influence of the central Pd(4) atom whose formal oxidation state is Pd(0).The sum of the two Pd(cent)-Si-C ipso angles and the C ipso -Si-C ipso angle is close to 360 • (355.5 • -360.0 • ).This suggests  12) and 2.4339(12) Å, respectively, which are considerably shorter than those observed in other Pd(edge)-Si moieties (2.5094(11)-2.6401(12)Å).In contrast, the Pd(edge)-Si(silylene) bond distances in Osakada's Pd 4 Si 3 cluster are reported to be 2.505-2.546Å with no significant deviation [35][36][37][38][39].This difference may be derived from the formal electron configurations of Pd (1) and Pd (2).As suggested by our previous theoretical calculations, the metal-to-silylene charge transfer, from the occupied d-orbital of the metal to the empty p-orbital of the silylene moieties, plays a crucial role in the bonding interaction between Pd(edge) and the bridging silylene moieties.Because Pd(3) has two isocyanide ligands, π-back donation from Pd to the two isocyanide ligands causes the Pd(3) centre to be more electron-deficient.This leads to longer Pd(3)-silylene bond distances compared with those of Pd(1)-Si(1) and Pd(2)-Si(1).The Pd(3)-Pd(4) bond length (2.7523(5) Å) is slightly longer than Pd(1)-Pd(4) (2.6812(6) Å) and Pd(2)-Pd(4) (2.6778(6) Å); however, they are within the range of metal-metal bonding interaction reported in the literature [10].Although there are three inequivalent palladium centres in the solid-state structure of 4, only one singlet 1 H NMR peak derived from the t Bu moiety of the isocyanide ligand was observed at 0.99 ppm in C6D6 at room temperature.In the variable temperature 1 H NMR spectrum of 4 in toluene-d8, this t Bu signal (appeared at 1.02 ppm at r.t.) started to broaden at around 0 °C, almost coalesced at −70 °C, then two signals appeared at 0.90 and 1.05 ppm at −90 °C.Similarly, methyl peaks corresponding to the i Pr moieties on the Si atoms also began to broaden at around 0 °C, then split into three slightly broad signals at −90 °C in an intensity ratio of ca.1:1:1.This suggests the presence of two inequivalent isocyanide ligands, as well as three inequivalent methyl groups of the Si i Pr2 moieties at lower temperatures.These spectral features suggest the presence of fluxional behaviour, due to the facile site exchange of the isocyanide ligands.The fluxional behaviour of the coordinated isocyanide ligands has also been observed in the previously reported transition metal clusters [40,41].The IR spectrum of the crystals of 4 displays two absorption bands at 2103 and 2065 cm −1 along with a shoulder band at 2125 cm −1 .This spectral feature is consistent with the solid-state structure determined by X-ray diffraction analysis.Unfortunately, no signal appeared in the 29 Si{ 1 H} NMR spectrum of 4 with longer (5 sec) or shorter (0.2 sec) relaxation time, even when using very concentrated sample with many scans.Although there are three inequivalent palladium centres in the solid-state structure of 4, only one singlet 1 H NMR peak derived from the t Bu moiety of the isocyanide ligand was observed at 0.99 ppm in C 6 D 6 at room temperature.In the variable temperature 1 H NMR spectrum of 4 in toluene-d 8 , this t Bu signal (appeared at 1.02 ppm at r.t.) started to broaden at around 0 • C, almost coalesced at −70 • C, then two signals appeared at 0.90 and 1.05 ppm at −90 • C. Similarly, methyl peaks corresponding to the i Pr moieties on the Si atoms also began to broaden at around 0 • C, then split into three slightly broad signals at −90 • C in an intensity ratio of ca.1:1:1.This suggests the presence of two inequivalent isocyanide ligands, as well as three inequivalent methyl groups of the Si i Pr 2 moieties at lower temperatures.These spectral features suggest the presence of fluxional behaviour, due to the facile site exchange of the isocyanide ligands.The fluxional behaviour of the coordinated isocyanide ligands has also been observed in the previously reported transition metal clusters [40,41].The IR spectrum of the crystals of 4 displays two absorption bands at 2103 and 2065 cm −1 along with a shoulder band at 2125 cm −1 .This spectral feature is consistent with the solid-state structure determined by X-ray diffraction analysis.Unfortunately, no signal appeared in the 29 Si{ 1 H} NMR spectrum of 4 with longer (5 sec) or shorter (0.2 sec) relaxation time, even when using very concentrated sample with many scans.

Ligand Exchange of 4 with N-Heterocyclic Carbene
In our previous paper, all 10 CN t Bu ligands in 2 were found to be easily and quantitatively replaced by another isocyanide ligand CN(2,4,6-Me 3 -C 6 H 2 ) to form a new "folding" Pd 11 cluster at −35 • C [29].Thus, the ligand exchange reaction of 4 with CN(2,4,6-Me 3 -C 6 H 2 ) was attempted.However, no reaction took place even at higher temperatures.Instead, 4 underwent the facile ligand exchange with 3 equiv.of N-heterocyclic carbene, i Pr 2 IM Me , to give the new planar Pd 4 Si 3 cluster Pd 4 {Si( i Pr) 2 } 3 ( i Pr 2 IM Me ) 3 (5) at room temperature (Scheme 3). 1 H NMR monitoring of this reaction indicated that 5 was formed in quantitative yield concomitant with the formation of free isocyanide, and it was isolated in 63% yield after purification.During this reaction, the four isocyanide ligands in 4 were replaced by three i Pr 2 IM Me ligands.Consequently, a more coordinatively unsaturated Pd 4 Si 3 cluster with 52 valence electrons was formed.An possible alternative synthetic route for cluster 5 may be the reaction of cyclotetrasilane 3 with "Pd( i Pr 2 IM Me ) 2 ".To check this possibility, we performed the reaction of 3 with "Pd( i Pr 2 IM Me ) 2 " which was generated in situ from the reaction of CpPd(η 3 -allyl) with 2 equiv. of i Pr 2 IM Me .We confirmed that no reaction took place even at higher temperatures such as 65 • C in C 6 D 6 , suggesting that cluster 5 is available only in the reaction shown in Scheme 3.
Inorganics 2017, 5, 84 5 of 12 In our previous paper, all 10 CN t Bu ligands in 2 were found to be easily and quantitatively replaced by another isocyanide ligand CN(2,4,6-Me3-C6H2) to form a new "folding" Pd11 cluster at −35 °C [29].Thus, the ligand exchange reaction of 4 with CN(2,4,6-Me3-C6H2) was attempted.However, no reaction took place even at higher temperatures.Instead, 4 underwent the facile ligand exchange with 3 equiv.of N-heterocyclic carbene, i Pr2IM Me , to give the new planar Pd4Si3 cluster Pd4{Si( i Pr)2}3( i Pr2IM Me )3 ( 5) at room temperature (Scheme 3). 1 H NMR monitoring of this reaction indicated that 5 was formed in quantitative yield concomitant with the formation of free isocyanide, and it was isolated in 63% yield after purification.During this reaction, the four isocyanide ligands in 4 were replaced by three i Pr2IM Me ligands.Consequently, a more coordinatively unsaturated Pd4Si3 cluster with 52 valence electrons was formed.An possible alternative synthetic route for cluster 5 may be the reaction of cyclotetrasilane 3 with "Pd( i Pr2IM Me )2".To check this possibility, we performed the reaction of 3 with "Pd( i Pr2IM Me )2" which was generated in situ from the reaction of CpPd(η 3 -allyl) with 2 equiv. of i Pr2IM Me .We confirmed that no reaction took place even at higher temperatures such as 65 °C in C6D6, suggesting that cluster 5 is available only in the reaction shown in Scheme 3. The molecular structure of 5 is shown in Figure 2, and selected bond distances are summarized in Table 1.Cluster 5 has a pseudo-C3 symmetric structure, and each Pd(edge) atom bears only one i Pr2IM Me ligand.Four Pd atoms and three Si(silylene) atoms are located on a plane, and deviations of these atoms from the plane are within the range of 0.031-0.178Å.As seen in cluster 4, all three Pd(4)-Pd(edge)-C( i PrIM Me ) axes deviate slightly from linearity (Pd(4)-Pd(1)-C( i PrIM Me ): 163.61(13)°,Pd(4)-Pd(2)-C( i PrIM Me ): 170.18(12)°,Pd(4)-Pd(3)-C( i PrIM Me ): 169.15(9)°).The dihedral angle between the Pd4Si3 plane and the five-membered ring of the i Pr2IM Me ligands are 87.5°-104.3°,suggesting the almost perpendicular orientation of the i Pr2IM Me moieties.Because cluster 5 has a pseudo-C3 symmetric structure, the three Pd(edge)-Pd(4) distances are essentially the same (2.6991(8)-2.7030(7)Å).Each silylene ligand bridges over two Pd(edge) atoms in an asymmetric manner.For instance, the bond distance of Pd(1)-Si(1) (2.3668(13) Å) is significantly shorter compared with that in Pd(2)-Si(1) (2.6359(14) Å).This may originate from the biased charge transfer from Pd to silylene moieties.Further studies including theoretical calculations to elucidate the details of the bonding interaction in the planar Pd4Si3 clusters are now underway.
The 1 H and 13 C{ 1 H} NMR spectra of 5 are consistent with the expectation from its pseudo-C3 symmetric structure.One singlet derived from the carbene carbon atom of the i Pr2IM Me moiety was observed at 194.0 ppm in the 13 C{ 1 H} NMR spectrum.The 29 Si{ 1 H} NMR spectrum of 4 showed a singlet at 223.1 ppm, which is comparable to those derived from the silylene moieties of 2 (191.85 and 226.63 ppm).This signal appeared at slightly lower field compared with Osakada's Pd4Si3 cluster (195 ppm) [35], but it is within the range for bridging silylene ligands reported in the literature [42][43][44].The molecular structure of 5 is shown in Figure 2, and selected bond distances are summarized in Table 1.Cluster 5 has a pseudo-C 3 symmetric structure, and each Pd(edge) atom bears only one i Pr 2 IM Me ligand.Four Pd atoms and three Si(silylene) atoms are located on a plane, and deviations of these atoms from the plane are within the range of 0.031-0.178Å.As seen in cluster 4, all three Pd(4)-Pd(edge)-C( i PrIM Me ) axes deviate slightly from linearity (Pd( 4

Ligand Exchange of 4 with Trimethylolpropane Phosphite to Afford the Planar Pd3 Cluster 6
The facile ligand exchange of 4 with i Pr2IM Me prompted us to examine the reaction of 4 with trimethylolpropane phosphite.This reaction was first monitored by 31 P{ 1 H} NMR.Treating 4 with 7 equiv. of trimethylolpropane phosphite provides two signals at 118.9 and 143.9 ppm.The former was assignable to the mononuclear Pd{P(OCH2)3CEt}4 complex by comparison with the independently prepared Pd{P(OCH2)3CEt}4 from the reaction of CpPd(η 3 -allyl) with 4 equiv. of P(OCH2)3CEt.With a lower amount of P(OCH2)3CEt, no intermediary species were visible in the reaction of 4 to form 6 and Pd{P(OCH2)3CEt}4.For example, treating 4 with 4 equiv. of P(OCH2)3CEt resulted in the exclusive formation of 6 and Pd{P(OCH2)3CEt}4 with recovery of 4. Because Pd{P(OCH2)3CEt}4 shows low solubility in diethyl ether or pentane, it can be easily removed from the crude product.Subsequent recrystallisation from pentane gave the planar tripalladium cluster 6 in a pure form.During this reaction, the sterically compact phosphite ligand may first attack the central palladium atom in 4, followed by the fragmentation to afford 6 and Pd{P(OCH2)3CEt}4 as products in 1:1 molar ratio (Scheme 4).It should be noted that the synthesis of structurally similar triplatinum clusters bearing bridging silylene ligands has been reported [45][46][47][48].However, to the best of our knowledge, cluster 6 is the first example of the palladium analogue of these clusters.The molecular structure of 6 was confirmed by X-ray diffraction analysis.The ORTEP drawing is shown in Figure 3, and the selected bond distances are depicted in Table 1.Cluster 6 can be regarded formally as a 42-electron cluster with three Pd-Pd bonds similar to those found in the previously reported trinuclear Pt3Si3 clusters [45][46][47][48].As shown in Figure 3b, there is a planar Pd3Si3 unit, and the deviations of all atoms from the plane are within the range of 0.000-0.172Å.One of the three phosphorus atoms (P(3) in Figure 3b) is located below the plane by 0.504 Å.The Pd-Pd bond  The facile ligand exchange of 4 with i Pr 2 IM Me prompted us to examine the reaction of 4 with trimethylolpropane phosphite.This reaction was first monitored by 31 P{ 1 H} NMR.Treating 4 with 7 equiv. of trimethylolpropane phosphite provides two signals at 118.9 and 143.9 ppm.The former was assignable to the mononuclear Pd{P(OCH 2 ) 3 CEt} 4 complex by comparison with the independently prepared Pd{P(OCH 2 ) 3 CEt} 4 from the reaction of CpPd(η 3 -allyl) with 4 equiv. of P(OCH 2 ) 3 CEt.With a lower amount of P(OCH 2 ) 3 CEt, no intermediary species were visible in the reaction of 4 to form 6 and Pd{P(OCH 2 ) 3 CEt} 4 .For example, treating 4 with 4 equiv. of P(OCH 2 ) 3 CEt resulted in the exclusive formation of 6 and Pd{P(OCH 2 ) 3 CEt} 4 with recovery of 4. Because Pd{P(OCH 2 ) 3 CEt} 4 shows low solubility in diethyl ether or pentane, it can be easily removed from the crude product.Subsequent recrystallisation from pentane gave the planar tripalladium cluster 6 in a pure form.During this reaction, the sterically compact phosphite ligand may first attack the central palladium atom in 4, followed by the fragmentation to afford 6 and Pd{P(OCH 2 ) 3 CEt} 4 as products in 1:1 molar ratio (Scheme 4).It should be noted that the synthesis of structurally similar triplatinum clusters bearing bridging silylene ligands has been reported [45][46][47][48].However, to the best of our knowledge, cluster 6 is the first example of the palladium analogue of these clusters.

Ligand Exchange of 4 with Trimethylolpropane Phosphite to Afford the Planar Pd3 Cluster 6
The facile ligand exchange of 4 with i Pr2IM Me prompted us to examine the reaction of 4 with trimethylolpropane phosphite.This reaction was first monitored by 31 P{ 1 H} NMR.Treating 4 with 7 equiv. of trimethylolpropane phosphite provides two signals at 118.9 and 143.9 ppm.The former was assignable to the mononuclear Pd{P(OCH2)3CEt}4 complex by comparison with the independently prepared Pd{P(OCH2)3CEt}4 from the reaction of CpPd(η 3 -allyl) with 4 equiv. of P(OCH2)3CEt.With a lower amount of P(OCH2)3CEt, no intermediary species were visible in the reaction of 4 to form 6 and Pd{P(OCH2)3CEt}4.For example, treating 4 with 4 equiv. of P(OCH2)3CEt resulted in the exclusive formation of 6 and Pd{P(OCH2)3CEt}4 with recovery of 4. Because Pd{P(OCH2)3CEt}4 shows low solubility in diethyl ether or pentane, it can be easily removed from the crude product.Subsequent recrystallisation from pentane gave the planar tripalladium cluster 6 in a pure form.During this reaction, the sterically compact phosphite ligand may first attack the central palladium atom in 4, followed by the fragmentation to afford 6 and Pd{P(OCH2)3CEt}4 as products in 1:1 molar ratio (Scheme 4).It should be noted that the synthesis of structurally similar triplatinum clusters bearing bridging silylene ligands has been reported [45][46][47][48].However, to the best of our knowledge, cluster 6 is the first example of the palladium analogue of these clusters.The molecular structure of 6 was confirmed by X-ray diffraction analysis.The ORTEP drawing is shown in Figure 3, and the selected bond distances are depicted in Table 1.Cluster 6 can be regarded formally as a 42-electron cluster with three Pd-Pd bonds similar to those found in the previously reported trinuclear Pt3Si3 clusters [45][46][47][48].As shown in Figure 3b, there is a planar Pd3Si3 unit, and the deviations of all atoms from the plane are within the range of 0.000-0.172Å.One of the three phosphorus atoms (P(3) in Figure 3b) is located below the plane by 0.504 Å.The Pd-Pd bond Scheme 4. Ligand exchange of 4 with trimethylolpropanephosphite to give the planar tripalladium cluster 6.
The molecular structure of 6 was confirmed by X-ray diffraction analysis.The ORTEP drawing is shown in Figure 3, and the selected bond distances are depicted in Table 1.Cluster 6 can be regarded formally as a 42-electron cluster with three Pd-Pd bonds similar to those found in the previously reported trinuclear Pt 3 Si 3 clusters [45 -48].As shown in Figure 3b, there is a planar Pd 3 Si 3 unit, and the deviations of all atoms from the plane are within the range of 0.000-0.172Å.One of the three phosphorus atoms (P(3) in Figure 3b) is located below the plane by 0.504 Å.The Pd-Pd bond distances are 2.7041(6)-2.7117(5)Å, which are almost comparable to those found in 5 and reported polynuclear palladium clusters [10].In contrast to clusters 4 and 5, the silylene ligands symmetrically bridge over two palladium atoms, and the Pd-Si bond lengths are 2.3557(9)-2.3754(11)Å, which are in the normal range for those found in palladium complexes having bridging silylene ligands.The NMR spectra of 6 are consistent with those expected from the molecular structure determined by X-ray diffraction analysis.For instance, the methyl and methine groups of the i Pr groups appeared at 1.86 and 1.93 ppm as doublet and septet, respectively.The 31 P{ 1 H} NMR spectrum of 6 exhibited an intense singlet at 143.9 ppm.
3.1.Synthesis of Pd 4 (Si( i Pr) 2 ) 3 (CN t Bu) 4 (4) In a 50-mL Schlenk tube, Pd(CN t Bu) 2 (273 mg, 1.00 mmol) was dissolved in toluene (20 mL), then octaisopropylcyclotetrasilane (3) (86 mg, 0.19 mmol) was added to this solution at room temperature.The solution was stirred at 65 • C for 18 h, then the solvent was removed in vacuo.The remaining crude product was dissolved in pentane (40 mL), and centrifuged to remove the insoluble materials.The supernatant was collected, concentrated to ca. 5 mL, and cooled at −35  (5) In a 20-mL Schlenk tube, 4 (67 mg, 0.061 mmol) was dissolved in toluene (20 mL), then i Pr 2 IM Me (33 mg, 0.183 mmol) was added to this solution at room temperature.The solution was stirred at room temperature for 1 h, then the solvent was removed in vacuo.The remaining solid was again dissolved in toluene (20 mL), stirred at room temperature for 1 h, then the solvent was removed in vacuo.The remaining crude product was extracted with pentane (10 mL), and the mother liquid was centrifuged to remove the small amount of insoluble materials.The supernatant was collected, then the solvent was removed in vacuo.The remaining powder was washed with HMDSO (5 mL × 2) to afford 5 as a brown powder (50 mg, 0.038 mmol, 63%).Crystals suitable for X-ray diffraction analysis were obtained by cooling the saturated pentane solution at −35  (6) In a 20-mL Schlenk tube, 4 (30 mg, 0.027 mmol) was dissolved in benzene (20 mL), then trimethylolpropane phosphite (31 mg, 0.19 mmol) was added to this solution at room temperature.The solution was stirred at room temperature for 1 h.After freeze-drying, the remaining solid was dissolved in ether (20 mL), and centrifuged to remove the insoluble materials.The supernatant was collected, then the solvent was removed in vacuo.The remaining powder was washed with HMDSO (5 mL) and cold pentane (3 mL) to afford 6 as a brown powder (16 mg, 0.014 mmol, 52%).Crystals suitable for X-ray diffraction analysis were obtained by cooling the saturated ether solution at −35 In a J. Young NMR tube, CpPd(η 3 -allyl) (20 mg, 0.094 mmol) was dissolved in C 6 D 6 (0.5 mL), then trimethylolpropane phosphite (61 mg, 0.38 mmol) was added to this solution at room temperature.The solution was allowed to stand at room temperature for 45 min. 1 H and 31 P{ 1 H} NMR spectra indicates quantitative formation of Pd{P(OCH 2 ) 3 CEt} 4 . 1 H NMR (400 MHz, r.t., C 6 D 6 ): δ = 0.03 (t, J = 7.6 Hz, 12H, CH 2 CH 3 ), 0.14 (q, 6H, J = 7.6 Hz, 8H, CH 2 CH 3 ), 3.65 (s, 24H, OCH 2 -). 31 P{ 1 H} NMR (162 MHz, r.t., C 6 D 6 ): δ = 118.9.

X-Ray Data Collection and Reduction
X-ray crystallography for complex 4 and 5 was performed on a Rigaku Saturn CCD area detector with graphite monochromated Mo Kα radiation (λ = 0.71075 Ǻ), and single crystals of 6 suitable for X-ray crystallography were analyzed by synchrotron radiation at beam line BL02B1 (λ = 0.71075 Ǻ) of Spring-8 (Hyogo, Japan) using Rigaku Mercury II detector.The data obtained were processed using Crystal-Clear (Rigaku) on a Pentium computer, and were corrected for Lorentz and polarization effects.The structures were solved by direct methods [51], and expanded using Fourier techniques [52].Hydrogen atoms were refined using the riding model.The final cycle of full-matrix least-squares refinement on F 2 was based on 12,489 observed reflections and 470 variable parameters for 4; 14,053 observed reflections and 577 variable parameters for 5; 11,457 observed reflections and 507 variable parameters for 6.Neutral atom scattering factors were taken from Cromer and Waber [53].All calculations were performed using the Crystal Structure [54] crystallographic software package except for refinement, which was performed using SHELXL-97 [55].Details of final refinement, as well as the bond lengths and angle, are summarized in Tables S1-S3, and the numbering scheme employed is also shown in Figures S7-S9, which were drawn with ORTEP at 50% probability ellipsoids.CCDC numbers 1582225 (4), 1582226 (5) and 1582227 (6) 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/data_request/cif.

Conclusions
In the present study, we found that the planar tetrapalladium cluster can be easily obtained by the reaction of cyclic tetrasilane with Pd(CN t Bu) 2 .The ligand exchange of the cluster led to the formation of new clusters, with or without maintaining the core structure.These results indicate that clustering metal atoms through insertion into the Si-Si bonds of cyclic organopolysilanes is an effective way to synthesise transition metal clusters with bridging organosilylene ligands.Efforts are underway to synthesise a series of new metal clusters by the reaction of appropriate transition metal precursors with cyclic organopolysilanes.Application of these cluster molecules as functional materials, such as catalysts, will also be investigated in the near future.

Supplementary Materials:
The following are available online at www.mdpi.com/2304-6740/5/4/84/s1,CIF and cif-checked files, detailed crystallographic data, the actual NMR charts of complexes 4, 5 and 6, and IR chart of 4. also supported by the special fund of Institute of Industrial Science, The University of Tokyo.The synchrotron radiation experiments were performed at the BL02B1 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2016A1118, 2016B1392, and 2017A1400).
Pd 4 Cluster 4 by Reaction of Pd(CN t Bu) 2 with Cyclotetrasilane 3

Figure 1 .
Figure 1.(a) Top view of the molecular structure of 4 with 50% probability ellipsoids.Hydrogen atoms were omitted for clarity; (b) Side view of 4. All atoms derived from the isocyanide ligands except for the coordinated carbon atoms, and all hydrogen atoms were omitted for clarity.

Figure 1 .
Figure 1.(a) Top view of the molecular structure of 4 with 50% probability ellipsoids.Hydrogen atoms were omitted for clarity; (b) Side view of 4. All atoms derived from the isocyanide ligands except for the coordinated carbon atoms, and all hydrogen atoms were omitted for clarity.
)-Pd(1)-C( i PrIM Me ): 163.61(13) • , Pd(4)-Pd(2)-C( i PrIM Me ): 170.18(12) • , Pd(4)-Pd(3)-C( i PrIM Me ): 169.15(9) • ).The dihedral angle between the Pd 4 Si 3 plane and the five-membered ring of the i Pr 2 IM Me ligands are 87.5 • -104.3 • , suggesting the almost perpendicular orientation of the i Pr 2 IM Me moieties.Because cluster 5 has a pseudo-C 3 symmetric structure, the three Pd(edge)-Pd(4) distances are essentially the same (2.6991(8)-2.7030(7)Å).Each silylene ligand bridges over two Pd(edge) atoms in an asymmetric manner.For instance, the bond distance of Pd(1)-Si(1) (2.3668(13) Å) is significantly shorter compared with that in Pd(2)-Si(1) (2.6359(14) Å).This may originate from the biased charge transfer from Pd to silylene moieties.Further studies including theoretical calculations to elucidate the details of the bonding interaction in the planar Pd 4 Si 3 clusters are now underway.The 1 H and 13 C{ 1 H} NMR spectra of 5 are consistent with the expectation from its pseudo-C 3 symmetric structure.One singlet derived from the carbene carbon atom of the i Pr 2 IM Me moiety was observed at 194.0 ppm in the 13 C{ 1 H} NMR spectrum.The 29 Si{ 1 H} NMR spectrum of 4 showed a singlet at 223.1 ppm, which is comparable to those derived from the silylene moieties of 2 (191.85 and

Figure 2 .
Figure 2. (a) Top view of the molecular structure of 5 with 50% probability ellipsoids.Hydrogen atoms were omitted for clarity; (b) Side view of 5.All atoms derived from the i Pr2IMMe ligands except for the coordinated carbon atoms, and all hydrogen atoms were omitted for clarity.

Figure 2 .
Figure 2. (a) Top view of the molecular structure of 5 with 50% probability ellipsoids.Hydrogen atoms were omitted for clarity; (b) Side view of 5.All atoms derived from the i Pr 2 IMMe ligands except for the coordinated carbon atoms, and all hydrogen atoms were omitted for clarity.

Figure 2 .
Figure 2. (a) Top view of the molecular structure of 5 with 50% probability ellipsoids.Hydrogen atoms were omitted for clarity; (b) Side view of 5.All atoms derived from the i Pr2IMMe ligands except for the coordinated carbon atoms, and all hydrogen atoms were omitted for clarity.
7041(6)-2.7117(5)Å, which are almost comparable to those found in 5 and reported polynuclear palladium clusters[10].In contrast to clusters 4 and 5, the silylene ligands symmetrically bridge over two palladium atoms, and the Pd-Si bond lengths are 2.3557(9)-2.3754(11)Å, which are in the normal range for those found in palladium complexes having bridging silylene ligands.

Figure 3 .
Figure 3. (a) Top view of the molecular structure of 6 with 50% probability ellipsoids.Hydrogen atoms were omitted for clarity; (b) Side view of 6.All atoms derived from the phosphite ligands except for the coordinated phosphorus atoms, and all hydrogen atoms were omitted for clarity.

4 5 6 Figure 3 .
Figure 3. (a) Top view of the molecular structure of 6 with 50% probability ellipsoids.Hydrogen atoms were omitted for clarity; (b) Side view of 6.All atoms derived from the phosphite ligands except for the coordinated phosphorus atoms, and all hydrogen atoms were omitted for clarity.