Steric and Electronic Effect of Cp-Substituents on the Structure of the Ruthenocene Based Pincer Palladium Borohydrides

Ruthenocene-based PCPtBu pincer ligands were used to synthesize novel pincer palladium chloride RcF[PCPtBu]PdCl (2a) and two novel palladium tetrahydroborates RcF[PCPtBu]Pd(BH4) (3a) and Rc*[PCPtBu]Pd(BH4) (3b), where RcF[PCPtBu] = κ3-{2,5-(tBu2PCH2)2-C5H2}Ru(CpF) (CpF = C5Me4CF3), and Rc*[PCPtBu] = κ3-{2,5-(tBu2PCH2)2C5H2}Ru(Cp*) (Cp* = C5Me5). These coordination compounds were characterized by X-ray, NMR and FTIR techniques. Analysis of the X-ray data shows that an increase of the steric bulk of non-metalated cyclopentadienyl ring in 3a and 3b relative to non-substituted Rc[PCPtBu]Pd(BH4) analogue (3c; where Rc[PCPtBu] = κ3-{2,5-(tBu2PCH2)2C5H2}Ru(Cp), Cp = C5H5) pushes palladium atom from the middle plane of the metalated Cp ring in the direction opposite to the ruthenium atom. This displacement increases in the order 3c < 3b < 3a following the order of the Cp-ring steric volume increase. The analysis of both X-ray and IR data suggests that BH4 ligand in both palladium tetrahydroborates 3a and 3b has the mixed coordination mode η1,2. The strength of the BH4 bond with palladium atom increases in the order Rc[PCPtBu]Pd(BH4) < Rc*[PCPtBu]Pd(BH4) < RcF[PCPtBu]Pd(BH4) that appears to be affected by both steric and electronic properties of the ruthenocene moiety.

The presence of a transition metal in the sandwich moiety allows varying the charge of the bimetallic complex and significantly affects the electrochemical characteristics of the chelated η 1 -metal center [23]. For example, in comparison with neutral precursors, the presence of a positive charge in bimetallic ruthenium-palladium complexes leads to a decrease in the electron density on the chelated palladium atom and contributes to an increase in its catalytic activity in Suzuki cross-coupling reactions [19,24,26]. It can be assumed that the introduction of electron-withdrawing groups into the sandwich core of the bimetallic complex can favorably affect the catalytic properties of the chelated metal atom. On the other hand, it is well known that in addition to the electronic factor, the catalytic activity of metal complexes is strongly affected by steric factors determining the spatial availability of the metal center for the substrate [8,15,16,31]. In the case of bimetallic PCP complexes of sandwich-type, the steric accessibility of the chelated metal atom is determined not only by the volume of organic groups R at the phosphorus donor atoms and the P-M-P angle but also by the presence of substituents in the non-metalated ring [17,23,25]. Thus, the presence of five methyl groups in the cyclopentadienyl ligand significantly increases its steric volume and donor ability in comparison with the unsubstituted Cp ring (Cp = η 5 -C 5 H 5 ). Replacing one of the methyl groups in the Cp* ligand (Cp* = η 5 -C 5 Me 5 ) with CF 3 group (Cp F = C 5 Me 4 CF 3 ) slightly increases the volume of the five-membered ring, but keeps the electronic effect of the ligand at the level of unsubstituted cyclopentadienyl [32][33][34][35].
An interesting feature of palladium chloride complexes based on ferrocene and ruthenocene is their ability to react with NaBH 4 to form the corresponding tetrahydroborate complexes Fc[PCP tBu ]Pd(BH 4 ) and Rc[PCP tBu ]Pd(BH 4 ). (where Fc[PCP tBu ] and Rc[PCP tBu ] = κ 3 -{2,5-( t Bu 2 PCH 2 ) 2 C 5 H 2 }M(Cp), M = Fe, Ru) [15,36]. Rc[PCP tBu ]Pd(BH 4 ) complex, which we have recently studied [36], proved to be a useful starting compound for the production of highly reactive palladium hydride Rc[PCP tBu ]PdH. However, despite a high potential of complexes of this type, the sterically loaded palladium pincer complexes remain almost unexplored. In this work, we report on the synthesis of two new palladium borohydride pincer complexes based on substituted ruthenocenes. The use of Cp* and Cp F ligands and comparison with the non-substituted Cp analogues allows accessing the steric and electronic effects of sandwich moiety on the properties of PCP tBu chelated palladium.

Synthesis
1,3-disubstituted ruthenocene bisphosphine with a Cp F ligand in the metallocene core {1,3-( t Bu 2 PCH 2 ) 2 C 5 H 2 }Ru(Cp F ) (1) was synthesized by phosphination of the corresponding diol {1,3-(HOCH 2 ) 2 C 5 H 2 }Ru(Cp F ) in acetic acid according to the previously published procedure [37]. 1,3-disubstituted ruthenocene bisphosphine with a Cp F ligand in the metallocene core {1,3-( t Bu2PCH2)2C5H2}Ru(Cp F ) (1) was synthesized by phosphination of the corresponding diol {1,3-(HOCH2)2C5H2}Ru(Cp F ) in acetic acid according to the previously published procedure [37]. As the reaction proceeds, a finely dispersed, unidentified, bright yellow precipitate and impurities are formed that significantly complicate the subsequent isolation of the target cyclometalation product. To facilitate the isolation of 2a, two equivalents of triethylamine were introduced into the reaction mixture after 3 h of reflux, which was continued for another 2 h. This approach led to the formation of a significant amount of palladium black, however, it allowed simplifying the purification procedure and gave the product with 31% yield. Note that the analogues palladium complex based on pentamethylruthenocene Rc * [PCP tBu ]PdCl (2b) (where Rc * [PCP tBu ] = κ 3 -{2,5-( t Bu 2 PCH 2 ) 2 C 5 H 2 }Ru(C 5 Me 5 )) was previously obtained under similar conditions with 30% yield. Apparently, the serious steric hindrance in the bisphosphine molecule created by the presence of five substituents in the cyclopentadienyl ring and tert-butyl groups at the donor phosphorus atoms hampers the cyclometalation reaction. For comparison, [PCP tBu ] complexes of palladium based on ferrocene, ruthenocene, and benzene can be obtained under comparable conditions with much higher yields [1,15,17]. It has been reported that in the preparation of benzene-based [POCOP tBu ] palladium complex the replacement of the most frequently used PdCl 2 (PhCN) 2 as a cyclometalating agent with palladium(II) chloride leads to an increase in the yield of the reaction product from 31 to 80% [38]. Unfortunately, the use of PdCl 2 as a cyclometalation reagent in the reaction with Rc F [PCP tBu ] (1) led to the formation of complex 2a only in trace amounts. Compound 2a is a pale yellow, air-stable powder. It was characterized by multinuclear NMR spectroscopy, and its purity was confirmed by elemental analysis.
The 31 P{ 1 H} and 19 F NMR spectra of complex 2a contain a single signal from two equivalent phosphorus nuclei and three equivalent fluorine nuclei at δ 82.07 ppm and −52.74 ppm, respectively. In the 1 H NMR, the proton signal of the metallated cyclopentadienyl ring is observed as a singlet at 4.22 ppm (2H). The signal of the six protons of two methyl groups located at the 2 ,5 positions of the Cp F ligand appears as a broadened multiplet at δ 1.89 ppm and the proton signals of two other methyl groups of the same ligand as a singlet at δ 1.77 ppm. Geminal methylene protons of CH 2 P t Bu 2 groups are not equivalent and appear as doublets of virtual triplets at δ 2. 45  The complete conversion of the starting chlorides 2 into borohydrides 3 can be reached in 8 h, adding extra NaBH 4 to the reaction mixture every hour. Monitoring of the formation of complex 3b using NMR spectroscopy showed that after 3 h of reflux the ratio of the reaction product to the starting compound is approximately 3:1. The formation of palladium hydride complexes was not observed. For comparison, the pincer chloropalladium complexes based on ferrocene Fc[PCP tBu ]PdCl [15] and ruthenocene Rc[PCP tBu ]PdCl [36] are completely converted to the corresponding tetrahydroborates in 2 h. Complexes 3a and 3b were isolated in analytically pure form with 90% and 93% yield, respectively, as bright yellow crystalline powders. They are stable at room temperature in the solid state but are sensitive to air and thermally unstable in solution. In chloroform, 3a and 3b rapidly transform into the chloropalladium precursors 2a and 2b. In general, sterically loaded borohydride complexes 3a and 3b are less stable than the previously obtained ruthenocene analogue Rc[PCP tBu ]Pd(BH 4 ) (3c) [36]. Compounds 3a and 3b were fully characterized by NMR, IR, and elemental analysis. The presence of BH 4 ligand coordinated to the palladium atom is confirmed by 1 H and 11 B NMR spectroscopy. The BH 4 protons appear in the 1 H NMR spectrum as a strongly broadened multiplet at δ H 0.13 and 0.21 ppm for 3a and 3b in C 6 D 6 , respectively. 11 B{ 1 H} NMR spectra exhibit broadened signals at δ −34.54 and −34.73, respectively. The broadening of the BH 4 group signals in the 1 H NMR spectra indicates a rapid averaging in solution of hydrogen atoms bound to boron. The 31 P{ 1 H} NMR spectra of complexes 3a and 3b exhibit singlets at δ P 87.44 and 87.07, respectively, indicating the equivalence of two phosphorus nuclei.
product from 31 to 80% [38]. Unfortunately, the use of PdCl2 as a cyclometalation reagent in the reaction with Rc F [PCP tBu ] (1) led to the formation of complex 2a only in trace amounts. Compound 2a is a pale yellow, air-stable powder. It was characterized by multinuclear NMR spectroscopy, and its purity was confirmed by elemental analysis.
The 31 P{ 1 H} and 19 F NMR spectra of complex 2a contain a single signal from two equivalent phosphorus nuclei and three equivalent fluorine nuclei at δ 82.07 ppm and −52.74 ppm, respectively. In the 1 H NMR, the proton signal of the metallated cyclopentadienyl ring is observed as a singlet at 4.22 ppm (2H). The signal of the six protons of two methyl groups located at the 2',5' positions of the Cp F ligand appears as a broadened multiplet at δ 1.89 ppm and the proton signals of two other methyl groups of the same ligand as a singlet at δ 1.77 ppm. Geminal methylene protons of CH2P t Bu2 groups are not equivalent and appear as doublets of virtual triplets at δ 2.45 and 2.51 ppm. Resonances of methyl protons of nonequivalent tert-butyl groups are observed in the form of two virtual triplets at δ 1.33 ppm and 1.47 ppm in accordance with the expected structure.
The  The complete conversion of the starting chlorides 2 into borohydrides 3 can be reached in 8 h, adding extra NaBH4 to the reaction mixture every hour. Monitoring of the formation of complex 3b using NMR spectroscopy showed that after 3 h of reflux the ratio of the reaction product to the starting compound is approximately 3:1. The formation of palladium hydride complexes was not observed. For comparison, the pincer chloropalladium complexes based on ferrocene Fc[PCP tBu ]PdCl [15] and ruthenocene Rc[PCP tBu ]PdCl [36] are completely converted to the corresponding tetrahydroborates in 2 h. Complexes 3a and 3b were isolated in analytically pure form with 90% and 93% yield, respectively, as bright yellow crystalline powders. They are stable at room temperature in the solid state but are sensitive to air and thermally unstable in solution. In Scheme 3. Synthesis of borohydrides 3a and 3b.

X-Ray Diffraction Study
The structures of complexes 3a and 3b are very similar as confirmed by single-crystal XRD analysis (Figures 1 and 2). They feature palladium atoms in a distorted square-planar environment with an angle P(1)-Pd(1)-P(2) equal to 155.67(2) and 158.87(5) • , respectively. Palladium atom deviates from the middle plane of the metalated Cp ring in the direction opposite to the ruthenium atom by 0.541(3) and 0.489(9) Å, respectively. For comparison, in the Rc[PCP tBu ]Pd(BH 4 ) complex (3c) the distance between the palladium atom and the plane of the Cp ring is 0.322 Å [36]. In both structures, the phosphorus atoms P(1) and P(2) rise above the Cp ring plane to different degrees: by 0.888(3) and 0.591(3) Å in case of 3a; 0.708(10) and 0.501(11) Å in case of 3b. The presence of five organic substituents in the non-metalated Cp ring makes the cyclopentadienyl rings noticeably non-coplanar. The angle between the planes of the cyclopentadienyl rings is 8.75(8) and 9.9(2) • in 3a and 3b, respectively. Similar values of this angle are reported for palladium chloride complex Rc*[PCP tBu ]PdCl (2b) [17].
It is known that the BH 4 anion can be coordinated to a transition metal atom in a monodentate (η 1 ), bidentate (η 2 ), or tridentate (η 3 ) fashion [39]. According to our X-ray diffraction data, in complexes 3a and 3b the BH 4 group is located in the syn position to the metallocene core relative to the Pd-C(1) bond line. The distance Pd-H(1) is 1.86(2) and 1.82(5) Å, respectively. These values significantly exceed the Pd-H bond length (1.53-1.75 Å) in the known palladium hydride pincer complexes [38,40,41] and are comparable to the values found for the ruthenocene-based complex Rc[PCP tBu ]Pd(BH 4 ) 3c (1.87(2) Å) [36]. The distance of the Pd atom to the second nearest hydrogen atom H(2) in structures 3a, 3b is 2. 46(2) and 2.34(6) Å, respectively (for comparison, d(Pd-H(2)) = 2.54(3) Å in 3c [36]) that is below the sum of Pd-H van der Waals radii. The difference between the distances Pd-H(1) and Pd-H(2) in 3a and 3b ∆d(Pd···H) = 0.60(2) and 0.52(5) Å is slightly lower than in 3c. These values are comparable to ∆d(Pd···H) in the previously described palladium borohydride complexes and fall in the reported range 0.2-0.67 Å [15,36,42,43]. An analysis of the above data votes for the monodentate η 1 type of BH 4 binding to palladium atom with more pronounced secondary Pd···H interaction relative to 3c. However, according to our DFT calculations for complex 3c the elongated Pd-H(2) contact makes an important additional contribution to the total binding of the BH 4 fragment to the palladium atom [36]. We believe  In complexes 3a and 3b, the Pd···B distance exceeds the sum of the covalent radii of Pd and B atoms (ca. 2.2 Å) being 2.531(2) and 2.560 (7) Å, respectively. For comparison, in complex 3c, the Pd···B distance is 2.587 (3)    In complexes 3a and 3b, the Pd···B distance exceeds the sum of the covalent radii of Pd and B atoms (ca. 2.2 Å) being 2.531(2) and 2.560 (7) Å, respectively. For comparison, in complex 3c, the Pd···B distance is 2.587  In complexes 3a and 3b, the Pd···B distance exceeds the sum of the covalent radii of Pd and B atoms (ca. 2.2 Å) being 2.531(2) and 2.560 (7) Å, respectively. For comparison, in complex 3c, the Pd···B distance is 2.587 Presumably, the additional (relative to complexes 3b and 3c) deviation of 4.4-4.7 • from linearity in 3a is due to the greater steric pressure of the bulky axial tert-butyl groups at phosphorus atoms on the borohydride ligand. Indeed, the distance between the central carbon atoms C(16) and C(21) of the axial tert-butyl groups in complexes 3a and 3b is 5.573(3) and 5.759(6) Å, respectively. For comparison, the same distance in complex 3c is 6.006(3) Å [36]. Thus, despite the bulky CF 3 group is in the remote position relative to Pd(BH 4 ) fragment, it affects the overall geometry of this pincer complex: the decreased inclination of two Cp rings puts three methyl groups close to tert-butyls pushing both tBu 2 P fragments above the plane of cyclometalated ring away from to the ruthenium atom. That, in turn, expels BH 4 moiety below the same plane decreasing the C(1)-Pd(1)-B(1) angle.

IR Spectra
Vibrational spectra are often used to determine the type of BH 4 group coordination to transition metal [39,44]. The IR spectra of complexes 3a and 3b are similar to the spectrum of the previously published Rc[PCP tBu ]Pd(BH 4 ) (3c) complex ( Figures S14-S17). In agreement with the crystallographic data, they indicate the presence of a secondary Pd···H interaction in addition to the primary Pd-H bond both in the crystalline state and in solution. In the spectra of solid samples of complexes 3a and 3b, the stretching vibration ν as (B-H term ) of the terminal B-H bonds is observed in the frequency range 2363-2388 cm −1 either as a split band (in case of 3a) or as a band with a shoulder (3b). Probably, the splitting is associated with the isotopic effect of 10 B-H/ 11 B-H [39] vibrations. In the CH 2 Cl 2 solution of complexes 3a and 3b, the same vibration appears as a non-split strong band at 2376 and 2372 cm −1 , respectively. Note that, in the series of complexes 3, a shift of this band to a higher frequency region is observed (Table 1), which may indicate an increase in the contribution of the bidentate form of BH 4 coordination [39,44]. The stretching bands ν s (B-H term ) of moderate intensity are observed in the IR spectrum of solid samples 3a and 3b at 2296 and 2291 cm −1 , respectively. The stretching vibrations ν(B-H bridge ) of a bridging hydrogen atom Pd-H-B in the spectra of complexes 3a, 3b appear as wide bands of moderate intensity at 2019 and 2025 cm −1 and a high-intensity band in the lower frequency region at 1838 and 1846 cm −1 , respectively. It should be noted that ν(B-H bridge ) stretching vibrations in the low-frequency region at 1700-2000 cm −1 are rarely observed in the IR spectra of M(η 1 -BH 4 ) complexes, but as a rule always present in the spectra of M(η 2 -BH 4 ) complexes [44]. The bands of bending vibrations δ(BH) in the spectra of complexes 3a, 3b are observed at 1054 and 1060 cm −1 , respectively, that is in the frequency range characteristic rather for the η 1 -bound BH 4 ligand [39,44].
The 1 H, 19 F, 11 B{ 1 H} NMR spectra were recorded on a Bruker Avance 400 spectrometer. 1 H chemical shifts are reported in ppm downfield to TMS using the residual signals of the solvent (CDCl 3 , δ 7.26; C 6 D 6 , δ 7.16) as the internal standard. The 19 F and 11 B{ 1 H} NMR spectra are given on the δ scale, the chemical shifts were measured relative to CFCl 3 and BF 3 ·Et 2 O, respectively, as external standards. 31 P{ 1 H} NMR spectra were recorded on Bruker Avance 300, or Bruker Avance 400 spectrometers and are reported in ppm using 85% H 3 PO 4 as external standard. 13 C{ 1 H} NMR spectra were recorded on a Bruker Avance 600 spectrometer with CDCl 3 (δ 77.16) or C 6 D 6 (δ 128.06) as the internal reference. FTIR spectra were measured on Shimadzu IR Prestige 21 FTIR spectrometer. Elemental analysis was performed at the Laboratory of Microanalysis of INEOS RAS. Single crystals of 3a and 3b were obtained by slow crystallization from toluene/hexane mixture. .0518] were used in a further refinement. Using Olex2 [45], the structures were solved with the ShelXT structure solution program [46] using Intrinsic Phasing and refined with the XL refinement package [47] using Least-Squares minimization. Hydrogen atoms of the BH groups were located from difference Fourier synthesis and refined freely in the isotropic approximation. Positions of all other atoms were calculated, and they were refined within the riding model. For 3a, the refinement converged to wR2 = 0.0535 and GOF = 1.023 for all the independent reflections (R1 = 0.0235 was calculated against F for 8034 observed reflections with I > 2σ(I)). For 3b, the refinement converged to wR2 = 0.0729 and GOF = 1.009 for all the independent reflections (R1 = 0.0394 was calculated against F for 8230 observed reflections with I>2σ(I)). CCDC 1993081 and 1993082 contain the supplementary crystallographic information for 3a and 3b, respectively. Crystal data and structure refinement parameters for 3a, 3b are summarized in Table S1. PdCl 2 (PhCN) 2 (270 mg, 0.703 mmol) was added to a suspension of bisphosphine 1 (470 mg, 0.700 mmol) in 2-methoxyethanol (20 mL). The mixture was refluxed with stirring for 3 h, then triethylamine (0.20 mL, 1.45 mmol) was rapidly added to the boiling solution using a syringe and the mixture was refluxed for additional 2 h. After cooling the resulting solution was diluted with dichloromethane (1:1) and filtered through celite. Then the solvents were removed in vacuo, and the residue was purified on an alumina column (eluent hexane-dichloromethane (2:1)). Recrystallization of the residue from ethanol gave pale-yellow solid of the product. Yield: 175 mg (31%). 1   NaBH 4 (100 mg, 2.63 mmol) was added to a solution of complexes 2a or 2b (110-120 mg) in 20 mL of ethanol-benzene (10:1) mixture at room temperature. The mixture was refluxed for 8 h, adding 50 mg (1.32 mmol) of NaBH 4 to the mixture every hour. The reaction mixture was then cooled and evaporated in vacuo to a minimum volume. Distilled water (20 mL) was added to the residue, the resulting suspension was stirred at room temperature for 1 h. The reaction mass was extracted (4 × 10 mL) with hexane-benzene (3:1). The solvents were evaporated in vacuo. Recrystallization of the residue from toluene-hexane (1:1) mixture gave solid products (3a or 3b), which were additionally dried in vacuo at room temperature.

Conclusions
Thus, our work demonstrated that the introduction of the bulky C 5 Me 4 CF 3 (Cp F ) and C 5 Me 5 (Cp*) ligands in the sandwich scaffold of the ruthenocene-based PCP tBu pincer palladium complexes does not preclude the formation of the corresponding palladium tetrahydroborate pincer complexes. The two novel sterically loaded pincer palladium tetrahydroborates Rc F [PCP tBu ]Pd(BH 4 ) (3a) and Rc*[PCP tBu ]Pd(BH 4 ) (3b) were synthesized and fully characterized by X-ray, NMR, and FTIR techniques. The X-ray diffraction study of 3a and 3b revealed that an increase of the steric bulk of non-metalated cyclopentadienyl ring in 3a and 3b relative to non-substituted Rc[PCP tBu ]Pd(BH 4 ) analogue (3c) pushes palladium atom from the middle plane of the metalated Cp ring in the direction opposite to the ruthenium atom. This displacement increases in the order 3c < 3b < 3a following the order of the Cp-ring steric volume increase. The analysis of both X-ray and IR data suggests that BH 4 ligand in both palladium tetrahydroborates 3a and 3b has the mixed coordination mode η 1,2 similar to that in 3c in which the primary Pd-H contact of 1.82-1.87Å is accompanied by the secondary interaction Pd···H of 2.34-2.54Å. Pd· · · B distances decrease in the order: Rc[PCP tBu ]Pd(BH 4 ) > Rc*[PCP tBu ]Pd(BH 4 ) > Rc F [PCP tBu ]Pd(BH 4 ) that suggests the increase in the strength of the BH 4 bond with the palladium atom that appears to be affected by both steric and electronic properties of the ruthenocene moiety.