Bimetallic rare-earth/platinum complexes ligated by phosphinoamides †

The heterometallic early-late 5d/4f binuclear phosphinoamido Ln/Pt(0) complexes [(Ph 2 PNHPh) Pt{µ-(Ph 2 PNPh)} 3 Ln(µ-Cl)Li(THF) 3 ] (Ln = Y ( 1a ), Lu ( 1b )) were obtained by reaction of [Li(THF) 4 ] [(Ph 2 PNPh) 4 Ln] (Ln = Y, Lu) with the Pt(0) complex [Pt( t Bu 3 P) 2 ] in the presence of LiCl. In the absence of LiCl the corresponding Ln/Pt(0) complexes [(Ph 2 PNHPh)Pt{µ-(Ph 2 PNPh)} 3 Ln{ η 2 -(Ph 2 PNPh)}][Li(THF) 4 ] (Ln = Y ( 2a ), Lu ( 2b )) were isolated. Both kind of complexes decompose in solution. The Pt(0) complex [Pt(Ph 2 PNHPh) 4 ] ( 3 ) was identi ﬁ ed as one of these decomposition products.


Introduction
Homo-and heterobimetallic complexes have been studied for catalytic application and small molecule activation over recent years. 1,2 Some of these compounds show cooperative and synergistic effects that can arise from the simultaneous or consecutive action of different metal centers in different media. 2 Inspired by nature, the concept of cooperative bimetallic catalysis has been employed for the synthesis of numerous artificial catalysts. The emerging field of homo-and heterobimetallic and polymetallic catalyst systems has been summarized in several review articles recently. [2][3][4][5][6] Within this area heterobimetallic early/late complexes 7,8 having interactions between a hard Lewis acidic early metal atom and a soft Lewis basic late metal atom are of interest because they feature significantly different reaction sites. This makes them interesting for potential applications in catalysis. 9 For the synthesis of heterobimetallic early/late complexes carbonyl-, hydride-, halide-, selenide-, and thiolate ligands were used as bridging ligands. 1 Recently, Thomas et al. reported heterobimetallic transition metal complexes with metal-to-metal bonds bridged by phosphinoamido ligands. 9 The known combinations of metals realized by this concept are bimetallic complexes, e.g. Co/Ti, 10,11 Co/Zr, 9,[12][13][14][15][16] Co/Hf, 17 Pt/Zr, 18 V/Fe, 19 Nb/Co, 20 Ta/Co, 20 and Co/U. 21 In contrast to transition metal complexes, heterobimetallic early/late complexes 7,8 containing rare-earth metals are far less common. Thus, only few complexes of the rare-earth elements with non-supported metal-to-metal bonds to a transition metal were reported. 17 Examples include complexes with Lu-Ru, 22 Ln-Re (Ln = La, Sm, Yb, Lu), 20,[23][24][25] Nd-Fe, 26 and Yb-Fe bonds. 27 Heterobimetallic compounds, which have a rare-earth metal atom and a rhodium, palladium, or platinum atom in close proximity (distance of less than 3.5 Å) are also not very common. Kempe et al. reported some Nd/Rh and Nd/Pd complexes, 28,29 in which the metals are brought closely together by bis(aminopyridinato) ligands. Hou et al. recently reported heterobimetallic rare-earth metal/platinum complexes. In these half-sandwich rare-earth metal alkyl complexes Cp ligands with a phosphine side arm were used. 30 As seen by these few examples one of the big challenges that still remain is the synthesis of heterobimetallic early/late complexes containing rare-earth metals. Lately, we reported the synthesis of the heterometallic early-late 4d/4f bi-and trinuclear phosphinoamido Ln/Pd(0) complexes [(Ph 2 PNHPh) Pd{μ-(Ph 2 PNPh)} 3 Ln(μ-Cl)Li(THF) 3 ] (Ln = Y (Ia), Lu (Ib)) and [Li(THF) 4  [(Ph 2 PNPh) 4 Ln] (Ln = Y, Lu) [33][34][35][36] with the palladium allyl complex [Pd 2 (C 3 H 5 ) 2 Cl 2 ]. A reduction of the palladium atoms was observed upon the formation of the bi-and trimetallic compounds Ia,b and IIa,b. Although the metal atoms are forced into close proximity by the phosphinoamido ligands, quantum chemical calculations of [(Ph 2 PNHPh)Pd{μ-(Ph 2 -PNPh)} 3 Lu(μ-Cl)Li(THF) 3 ] showed only weak metal-to-metal interactions. 31,32 Motivated by these initial results, we were interested in extending our studies on heterobimetallic early/late complexes containing rare-earth metals. Herein, we now report heterobimetallic rare-earth metal/platinum complexes bridged by phosphinoamido ligands.

Results and discussion
The palladium complexes Ia,b were obtained most efficiently by the addition of LiCl to the reaction mixture giving the desired complexes. In a similar protocol, the reaction of , Lu (1b)) in moderate yields (Scheme 2). Formally, the Pt atom inserts into the weak Ln-P bonds forming the desired heterobimetallic complexes. We presume that some decomposition occurs and Ph 2 PN(H)Ph, which is coordinated to the Pt atom, is formed as a side-product. In contrast to the formation of the Pd(0) complexes Ia,b, which were obtained only in a reductive approach from a Pd(II) source, compounds 1a,b are directly accessible from a Pt(0) precursor. By using Pt(II) salts as starting material no traceable products were obtained.
Although the reactions leading to 1a and 1b seem, at first glance, quiet similar, their accessibility is significantly different. Whereas 1a was obtained straight forward in a reproducible way, the preparation of 1b is more difficult.
Single crystals of 1a,b suitable for X-ray diffraction were obtained by crystallization from THF/toluene/pentane ( Fig. 1 and S1 †). Compounds 1a,b crystallize in the triclinic space group P1 with one molecule of the complexes in the asymmetric unit. Furthermore, one molecule of THF and toluene were localized each in the asymmetric unit. The toluene molecule in 1a showed a strong disorder and was thus suppressed by using Olex solvent mask. 37 In both compounds, the Ln and the Pt atoms are forced in close proximity by three bridging µ-(Ph 2 PNPh) ligands. As expected the soft P atom binds to the Pt atom, whereas the hard nitrogen atoms coordinate to the rare-earth metal atom. In both compounds, the Li atom is coordinated to the Ln atom via a µ-Cl bridge. The rare-earth atoms are thus five-fold coordinated by three Ph 2 PNPh ligands, one molecule of THF and the chlorine atom. A distorted trigonal bipyramidal coordination polyhedron with the THF oxygen atom and N3 in the axis is formed. The Ln-N bond distances (1a: 2.291(3) Å-2.347(3); 1b: 2.242(6)-2.297(6) Å) are in the range of Ia,b (av. 2.317 Å (Ia), 2.262 Å (Ib)). 31 The Ln-Cl bond lengths are 2.6525(14) Å (1a), 2.601(2) Å (1b)). The Pt atom is four-fold coordinated by the phosphorous atoms of three μ-(Ph 2 PNPh) ligands and one Ph 2 PNHPh ligand. A distorted tetrahedral coordination polyhedron is formed by the four P atoms around the Pt atom. The Pt-P bond distances are av. 2.3354 Å (1a), 2.328 Å (1b). Although the Pt atom has a slightly larger van-der-Waals radius in comparison to Pd, the Ln-Pt distances in 1a,b (3.0063(8) Å (1a), 2.9523(9) Å (1b)) are in the range of those in the Ln-Pd complexes Ia,b (2.9898(6) Å (Ia), 2.9031(11) Å (Ib)). This is clearly showing that the three μ-(Ph 2 PNPh) ligands are forcing the metal atoms into close proximity.
Since  [(Ph 2 PNPh) 4 Ln]. [Li(THF) 4 ][(Ph 2 PNPh) 4 Ln] 33-36 is obtained by the reaction of LnCl 3 with LiPPh 2 NPh in a 1 : 4 molar ratio. Usually, the product can be directly obtained by crystallization from THF/n-pentane. But obviously the bulk material is sometimes contaminated with traces of LiCl. By extraction of the crude product with toluene before crystallization, the contamination of the product with LiCl is avoided. The desired product is thus obtained in higher purity.
Reaction As seen for the formation of compounds 1a,b, there is a difference in reactivity. Upon workup, 2a was obtained as pure material in single crystalline form and complete characterization was possible. In contrast, the reaction leading to 2b is not quantitative. Even after prolonged reaction times the workup of 2b resulted in a mixture of the desired product and [Li(THF) 4 ][(Ph 2 PNPh) 4 Lu]. The solid-state structures of both 2a and 2b were established by single crystal X-ray diffraction but for 2b no further analytical data could be collected.
Compound 2a crystallizes in the monoclinic space group Cc with one molecule of the complexes in the asymmetric unit. Although the X-ray data collected from 2b was very poor, its composition was deduced from the difference Fourier map (Fig. S2 †). Bond angles and distances of 2b thus are not discussed. Compounds As observed for Ia,b, also the Pt complexes 1a,b and 2a,b decompose in solution. This is one explanation for the low yields and the formation of the protonated ligand PPh 2 NHPh. The 31 P{ 1 H} NMR spectra show fast decomposition of all compounds in various solvents. In d 8 -THF a large number of signals were observed. In C 6 D 6 major signals with the corresponding 195 Pt satellites were observed in the 31 P{ 1 H} NMR spectra of 1a,b and 2a (Fig. S3-S5 †). However, the ratio varies from sample to sample and further decomposition signals were sometimes also observed as well. As a result of the fast decomposition, there remain significant uncertainties of the correct assignment of the signals. A 1 H, 195 Pt HMBC NMR spectrum was also not conclusive.
One of the decomposition products could be identified. From a saturated solution of 1a in d 8 -THF the Pt(0) complex [Pt(PPh 2 NHPh) 4 ] (3) crystallized once in a NMR tube. Although 3 was not fully characterized and the X-ray data collected was poor, its composition was deduced from the difference Fourier map (Fig. 3) giving thus some insight into the decomposition pathway. Moreover, 3 was also identified by ESI-MS   spectroscopy of a solution of 1a. In 3, the Pt(0) atom is fourfold coordinated by the phosphorous atoms of four PPh 2 NHPh ligands in a tetrahedral fashion. The only slightly related phosphinoamido structure of platinum reported in the literature is found in the Pt(I) species [Pt(NPhPPh 2 )(HNPhPPh 2 )] 2 , having a Pt-Pt bond. 38

Experimental 32
All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in Schlenk-type glassware either on a dual manifold Schlenk line, interfaced to a high vacuum (10 −3 torr) line, or in an argonfilled MBraun glove box. Elemental analyses were carried out with an Elementar vario Micro Cube. Hydrocarbon solvents were predried using an MBraun solvent purification system (SPS-800) and then they were degassed, dried and stored in vacuo over LiAlH 4 . Tetrahydrofuran was distilled under nitrogen from potassium before storage over LiAlH 4 . Deuterated solvents were obtained from Euro-Isotop (99.5 atom% D) and were degassed, dried and stored in vacuo over Na/K alloy in resealable flasks. 1 H and 31 P{ 1 H} NMR spectra were recorded on a Bruker Avance II 300 MHz or Avance 400 MHz. 1 H chemical shifts were referenced to the residual deuterated solvents and are reported relative to tetramethylsilane, 31 P{ 1 H} was referenced to external 85% phosphoric acid. IR spectra were obtained on a Bruker Tensor 37 FTIR spectrometer equipped with a room temperature DLaTGS detector and a diamond ATR (attenuated total reflection) unit. Raman spectra were recorded on a Bruker MultiRAM spectrometer. 195 Pt-NMR spectra were recorded at 300 K on a Bruker Avance II 600 spectrometer using a double-resonance 1 H-BBI probe head. Platinum frequencies were determined by an ultrabroadband version of a gradient selected 1 H, 195 Pt-HMBC. 39 Due to the very large chemical shift range of platinum complexes (15 000 ppm, 1.94 MHz @ 14.1 T), it is not possible to cover this range in one conventional experiment. Conventional experiments are acquired with hard pulses that can excite bandwidth of about 50 kHz. Broadband spectra are achieved via application of broadband saturation pulses on platinum, which have been designed by optimal control derived optimizations. [40][41][42][43][44][45] [Pt(P(tBu) 3 4 Lu] and 53 mg (0.089 mmol) bis(tri-tert-butylphosphine)platinum(0). The mixture was stirred for 2 days at ambient temperature. After the reaction period the solution was concentrated and 10 ml of toluene was condensed onto the mixture. After filtration the THF/toluene mixture was layered with toluene and with n-pentane. After two times of recrystallization yellow (2b) and colourless crystals of [Li(THF) 4 ][(PPh 2 NPh) 4 Lu] could be obtained, which could not be separated from each other.
X-Ray-crystallographic studies of 1a, 1b, 2a, 2b and 3 A suitable crystal was covered in mineral oil (Aldrich) and mounted on a glass fiber. The crystal was transferred directly to the cold stream of a STOE IPDS 2 or STOE StadiVari diffractometer.
All structures were solved using SHELXS-2013. 48 The remaining non-hydrogen atoms were located from difference Fourier map calculations. The refinements were carried out by using full-matrix least-squares techniques on F, minimizing the function (F o − F c ) 2 , where the weight is defined as 4F o 2 / 2(F o 2 ) and F o and F c are the observed and calculated structure factor amplitudes using the program SHELXL-2013. 48 Carbonbound hydrogen atom positions were calculated. The locations of the largest peaks in the final difference Fourier map calculation as well as the magnitude of the residual electron densities in each case were of no chemical significance. Positional parameters, hydrogen atom parameters, thermal parameters, bond lengths and angles have been deposited as ESI. † Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as a supplementarypublication no. CCDC 1450174-1450176. Crystal