Synthesis and Characterization of Phosphinecarboxamide and Phosphinecarbothioamide, and Their Complexation with Palladium(II) Complex

Reactions of isocyanates/isothiocyanates with primary and secondary phosphines without solvent at room temperature afforded phosphinecarboxamide/phosphinecarbothioamide, respectively, in excellent yields. Furthermore, palladium complex Pd(COD)Cl2 was allowed to react with Ph2PC(O)NHPh (1a) to afford [Pd{Ph2PC(O)NHPh-κP}2Cl2] (3). On the other hand, the reaction of Pd(COD)Cl2 with 1 eq. of Ph2PC(S)NHPh (2a) afforded [PdCl2{Ph2PC(S)NHPh-κP,S}] (4). In the case of a 1:2 molar ratio, [PdCl{Ph2PC(S)NHPh-κP,S}{Ph2PC(S)NHPh-κP}]Cl (5) was formed. The newly obtained compounds were fully characterized using multielement NMR measurements and elemental analyses. In addition, the molecular structures of Ph2PC(O)NH(CH2)2Cl (1j), Ph2PC(S)NHPh(4-Cl) (2c), and 3–5 were determined using single-crystal X-ray diffraction.


Introduction
Phosphinecarboxamides R 2 PC(O)NR 2 and phosphinecarbothioamides R 2 PC(S)NR 2 are phosphorus-analogs of urea and thiourea, and these are interesting compounds in coordination chemistry because of the variety of coordination modes. These compounds are known to act as a P-coordinated monodentate ligand (type I) [1-7], a P,S-coordinated bidentate ligand forming a four-membered chelate ring (type II for phosphinecarbothioamide) [8][9][10][11][12][13], and a bridging ligand for two metal centers (type III and type IV for phosphinecarbothioamide) [7,14] (Figure 1).

Introduction
Phosphinecarboxamides R2PC(O)NR′2 and phosphinecarbothioamides R2PC(S)NR′2 are phosphorus-analogs of urea and thiourea, and these are interesting compounds in coordination chemistry because of the variety of coordination modes. These compounds are known to act as a P-coordinated monodentate ligand (type I) [1-7], a P,S-coordinated bidentate ligand forming a four-membered chelate ring (type II for phosphinecarbothioamide) [8][9][10][11][12][13], and a bridging ligand for two metal centers (type III and type IV for phosphinecarbothioamide) [7,14] (Figure 1). NR  One of the most efficient methods for the synthesis of phosphinecarboxamide/phosphinecarbothioamide is the hydrophosphination of isocyanates/isothiocyanates. The reaction without a metal catalyst has some drawbacks (low yield and long reaction time) One of the most efficient methods for the synthesis of phosphinecarboxamide/ phosphinecarbothioamide is the hydrophosphination of isocyanates/isothiocyanates. The reaction without a metal catalyst has some drawbacks (low yield and long reaction time) [15]. Therefore, metal-mediated methods have been developed, and there have been two reports on rare earth metals (La, Y, Eu, Er, Yb) [16,17], two reports on Th and U [18,19], one report on Fe [20] and one on Zn [21] for both isocyanates and isothiocyanates, and one report each on Zr [22], Cu [23] and Sb [24] for isocyanates to date. However, the removal of toxic metals after the end of the reaction is required when a metal compound is used. Therefore, the development of metal-free methods has been researched.
Recently, the transition-metal complex bearing 1,1 -bis(diphenylphosphinecarboxamidyl) ferrocene Fc(NHC(O)PPh 2 ) 2 as a ligand has been reported to exhibit unique properties. The tetranuclear Pt complex with six Fc(NHC(O)PPh 2 ) 2 has a hollow cage structure [25], and the macrocyclic dimer of Fc(NHC(O)PPh 2 -AuCl) 2 , induced by aurophilic interactions, shows helical chirality induction into the ferrocene core when using chiral proline methyl ester hydrochloride [26]. Although phosphinecarboxamide has attracted attention as a ligand, no examples have been reported comparing the ligand behavior of phosphinecarboxamide and its thiocarbonyl analog (phosphinecarbothioamides) with the same transition-metal complex precursor. In this study, we report the synthesis of isocyanate/isothiocyanate without using a transition-metal catalyst and the reactions of Pd(COD)Cl 2 (COD = η 2 :η 2 -1,5-cyclooctadiene) being a typical Pd(II) complex with Ph 2 PC(E)NHPh (E = O, S). We found the first example of the thermal cis/trans isomerization of a transition-metal complex with phosphinecarboxamide and phosphinecarbothioamide having intra-and/or inter-molecular hydrogen bonds. A part of this work was preliminarily reported in [27].

Synthesis of Phosphinecarboxamide and Phosphinecarbothioamide
Previously, we found that iron catalyzes the hydrophosphination of the unsaturated C-C bonds of alkynes and vinylphosphines with secondary phosphines [28,29]. In these reactions, phosphine compounds do not act as a catalyst poison. Therefore, we thought that the hydrophosphination reaction of isocyanates and isothiocyanates may be adaptable in our system. We performed the reaction-condition screening of the hydrophosphination of phenylisocyanates and found that no solvent was the key reaction condition. The desired hydrophosphination product, Ph 2 PC(O)NHPh 1a, was obtained within 30 min in >99% yield at room temperature when catalyst and solvent were not used Scheme 1.
Molecules 2022, 27, x FOR PEER REVIEW 2 of 13 [15]. Therefore, metal-mediated methods have been developed, and there have been two reports on rare earth metals (La, Y, Eu, Er, Yb) [16,17], two reports on Th and U [18,19], one report on Fe [20] and one on Zn [21] for both isocyanates and isothiocyanates, and one report each on Zr [22], Cu [23] and Sb [24] for isocyanates to date. However, the removal of toxic metals after the end of the reaction is required when a metal compound is used. Therefore, the development of metal-free methods has been researched. Recently, the transition-metal complex bearing 1,1′-bis(diphenylphosphinecarboxamidyl)ferrocene Fc(NHC(O)PPh2)2 as a ligand has been reported to exhibit unique properties. The tetranuclear Pt complex with six Fc(NHC(O)PPh2)2 has a hollow cage structure [25], and the macrocyclic dimer of Fc(NHC(O)PPh2-AuCl)2, induced by aurophilic interactions, shows helical chirality induction into the ferrocene core when using chiral proline methyl ester hydrochloride [26]. Although phosphinecarboxamide has attracted attention as a ligand, no examples have been reported comparing the ligand behavior of phosphinecarboxamide and its thiocarbonyl analog (phosphinecarbothioamides) with the same transition-metal complex precursor. In this study, we report the synthesis of isocyanate/isothiocyanate without using a transition-metal catalyst and the reactions of Pd(COD)Cl2 (COD = η 2 :η 2 -1,5-cyclooctadiene) being a typical Pd(II) complex with Ph2PC(E)NHPh (E = O, S). We found the first example of the thermal cis/trans isomerization of a transition-metal complex with phosphinecarboxamide and phosphinecarbothioamide having intra-and/or inter-molecular hydrogen bonds. A part of this work was preliminarily reported in [27].

Synthesis of Phosphinecarboxamide and Phosphinecarbothioamide
Previously, we found that iron catalyzes the hydrophosphination of the unsaturated C-C bonds of alkynes and vinylphosphines with secondary phosphines [28,29]. In these reactions, phosphine compounds do not act as a catalyst poison. Therefore, we thought that the hydrophosphination reaction of isocyanates and isothiocyanates may be adaptable in our system. We performed the reaction-condition screening of the hydrophosphination of phenylisocyanates and found that no solvent was the key reaction condition. The desired hydrophosphination product, Ph2PC(O)NHPh 1a, was obtained within 30 min in >99% yield at room temperature when catalyst and solvent were not used Scheme 1. We examined the substrate scope and limitation for the hydrophosphination of isocyanates with diphenylphosphine (Scheme 2). Phenylisocyanates with an electron-withdrawing group such as F, Cl, Br, and CF3 at the para position and Cl at the meta position on the phenyl ring were readily converted (within 40 min) into the corresponding phosphinecarboxamides in excellent yields (1b, 1c, 1d, 1e, 1h). The reaction of para-methoxyphenylisocyanate and that of 1-naphthylisocyanate took longer (1f, 1g). Isocyanates with aliphatic substituent were also adaptable to this reaction (1i and 1j). When 2-chlorophenylisocyanate, sterically bulky cyclohexylisocyanate, and 1-adamantylisocyanate were used, the desired products were not yielded. The double hydrophosphination of 1,3and 1,4-diisocyanatobenzene were found to yield the desired products, 1k and 1l, in 85% and 92% yields, respectively. In addition, the triple hydrophosphination product 1m could be obtained in 64% yield. Scheme 1. Hydrophosphination reaction of phenylisocyanate with diphenylphosphine.
We examined the substrate scope and limitation for the hydrophosphination of isocyanates with diphenylphosphine (Scheme 2). Phenylisocyanates with an electronwithdrawing group such as F, Cl, Br, and CF 3 at the para position and Cl at the meta position on the phenyl ring were readily converted (within 40 min) into the corresponding phosphinecarboxamides in excellent yields (1b, 1c, 1d, 1e, 1h). The reaction of paramethoxyphenylisocyanate and that of 1-naphthylisocyanate took longer (1f, 1g). Isocyanates with aliphatic substituent were also adaptable to this reaction (1i and 1j). When 2chlorophenylisocyanate, sterically bulky cyclohexylisocyanate, and 1-adamantylisocyanate were used, the desired products were not yielded. The double hydrophosphination of 1,3and 1,4-diisocyanatobenzene were found to yield the desired products, 1k and 1l, in 85% and 92% yields, respectively. In addition, the triple hydrophosphination product 1m could be obtained in 64% yield. The results of the scope and limitation for the hydrophosphination of phenylisocyanate with phosphine compounds are summarized in Scheme 3. Diphenylphosphine analogs with electron-donating or -withdrawing groups such as Me, OMe, and F at the para position of the phenyl rings, and dialkylphosphines such as t Bu2PH and i Pr2PH were also adaptable to this reaction. In the case of primary phosphine PhPH2, single hydrophosphination product 1s was obtained in 71%, but a long reaction time (3 days) was required because of the small amount of hexane existing in commercially available PhPH2 (see Section 3.1). The results of the scope and limitation for the hydrophosphination of phenylisocyanate with phosphine compounds are summarized in Scheme 3. Diphenylphosphine analogs with electron-donating or -withdrawing groups such as Me, OMe, and F at the para position of the phenyl rings, and dialkylphosphines such as t Bu 2 PH and i Pr 2 PH were also adaptable to this reaction. In the case of primary phosphine PhPH 2 , single hydrophosphination product 1s was obtained in 71%, but a long reaction time (3 days) was required because of the small amount of hexane existing in commercially available PhPH 2 (see Section 3.1). The results of the scope and limitation for the hydrophosphination of phenylisocyanate with phosphine compounds are summarized in Scheme 3. Diphenylphosphine analogs with electron-donating or -withdrawing groups such as Me, OMe, and F at the para position of the phenyl rings, and dialkylphosphines such as t Bu2PH and i Pr2PH were also adaptable to this reaction. In the case of primary phosphine PhPH2, single hydrophosphination product 1s was obtained in 71%, but a long reaction time (3 days) was required because of the small amount of hexane existing in commercially available PhPH2 (see Section 3.1). We reported the hydrophosphination of phenylisothiocyanate with diphenylphosphine to give the corresponding phosphinecarbothioamide 2a in a previous study [27]. Very recently, the hydrophosphination of isothiocyanates using a catalyst-free method was reported by Zhu and Wang's group [30]. For comparison, we also examined the synthesis of phosphinecarbothioamide without catalyst and solvent. Scheme 4 summarizes the results showing good functional group tolerance. Several isothiocyanates could be employed in the reaction, in which R is the phenyl ring with withdrawing groups such as F, Cl, Br, and CF 3 at the para position (2a, 2b, 2c, 2d, 2e). These reactions were completed within 30 min. Using 4-methoxyphenylisothiocyanate, 1-naphthylisothiocyanate and 3-chlorophenylisocyanate, a long reaction time was required (2f, 2g, 2h). Surprisingly, the hydrophosphination reaction of 2-chlorophenylisothiocyanate took place. This result is in contrast with the hydrophosphination of 2-chlorophenylisocyanate. Isothiocyanate with an aliphatic substituent was also adaptable to this reaction (2j). 1-Adamathylisothiocyanate did not react. These tendencies are similar to those observed for the corresponding isocyanates except for 2-chlorophenylisocyanate. The hydrophosphination of phenylisothiocyanate with (4-methylphenyl) 2 PH also proceeded well (2k). We reported the hydrophosphination of phenylisothiocyanate with diphenylphosphine to give the corresponding phosphinecarbothioamide 2a in a previous study [27]. Very recently, the hydrophosphination of isothiocyanates using a catalyst-free method was reported by Zhu and Wang's group [30]. For comparison, we also examined the synthesis of phosphinecarbothioamide without catalyst and solvent. Scheme 4 summarizes the results showing good functional group tolerance. Several isothiocyanates could be employed in the reaction, in which R is the phenyl ring with withdrawing groups such as F, Cl, Br, and CF3 at the para position (2a, 2b, 2c, 2d, 2e). These reactions were completed within 30 min. Using 4-methoxyphenylisothiocyanate, 1-naphthylisothiocyanate and 3-chlorophenylisocyanate, a long reaction time was required (2f, 2g, 2h). Surprisingly, the hydrophosphination reaction of 2-chlorophenylisothiocyanate took place. This result is in contrast with the hydrophosphination of 2-chlorophenylisocyanate. Isothiocyanate with an aliphatic substituent was also adaptable to this reaction (2j). 1-Adamathylisothiocyanate did not react. These tendencies are similar to those observed for the corresponding isocyanates except for 2-chlorophenylisocyanate. The hydrophosphination of phenylisothiocyanate with (4-methylphenyl)2PH also proceeded well (2k).  The molecular structures of 1i and 2c were determined using a single-crystal X-ray diffraction study. Two independent molecules of 1i were crystallized in the unsymmetric unit. The ORTEP drawings are shown in Figure 2a) for 1i (P1 molecule) and b) for 2c, along with selected bond lengths and angles. The bond distances of P1-C3 (1.870(2) Å) and C3-N1 (1.328(3) Å) for 1i were similar to those of P1-C7 (1.8541(17) Å) and N1-C7 (1.336(2) Å) for 2c. The phosphinecarboxamide and phosphinecarbothioamide moieties displayed a pyramidalized geometry at the P atom (sum of angle around P1 = 330.8° for 1i and 306.5° for 2c), indicating that the π-electron conjugation of the C=O and C=S did not extend to the P. In contrast, the planar carboxamide and carbothioamide moieties (sum of angle around N1 = 359.9° for 1i and 360.0° for 2c) were observed, showing the delocalization of π-electron density between the carbonyl (thiocarbonyl) and amide groups. These were consistent with the previously reported phosphinecarboxamide [31]. The molecular structures of 1i and 2c were determined using a single-crystal X-ray diffraction study. Two independent molecules of 1i were crystallized in the unsymmetric unit. The ORTEP drawings are shown in Figure 2a) for 1i (P1 molecule) and b) for 2c, along with selected bond lengths and angles. The bond distances of P1-C3 (1.870(2) Å) and C3-N1 (1.328(3) Å) for 1i were similar to those of P1-C7 (1.8541(17) Å) and N1-C7 (1.336(2) Å) for 2c. The phosphinecarboxamide and phosphinecarbothioamide moieties displayed a pyramidalized geometry at the P atom (sum of angle around P1 = 330.8 • for 1i and 306.5 • for 2c), indicating that the π-electron conjugation of the C=O and C=S did not extend to the P. In contrast, the planar carboxamide and carbothioamide moieties (sum of angle around N1 = 359.9 • for 1i and 360.0 • for 2c) were observed, showing the delocalization of π-electron density between the carbonyl (thiocarbonyl) and amide groups. These were consistent with the previously reported phosphinecarboxamide [31]. It is noteworthy that gram-scale practical reactions were successfully perfor phosphinecarboxamide 1a and phosphinecarbothioamide 2a were isolated in 90% 99% yields, respectively (Scheme 5).

Synthesis of Palladium(II) Complexes with Phosphinecarboxamide and Phosphinecarbothioamide
The reaction of Pd(COD)Cl2 with 1a at a 1:2 molar ratio produced the re bis(phosphinecarboxamide)palladium complex [Pd{Ph2PC(O)NHPh-κP}2Cl2] (3) in yield. Complex 3 was obtained even when Pd(COD)Cl2 was treated with 1 equiv. (NMR yield: 48%) (Scheme 6). The NH proton signal of 1a was not observed in C because of the overlapped phenyl protons, but it was observed at δ 7.13 ppm in C6D6 In the 1 H NMR spectrum of 3 in CDCl3, the NH proton was observed at δ 10.56 ppm. chemical shift indicates that the NH of 3 is involved in hydrogen bonding. The 31 P N spectrum of 3 showed two singlets (δ 25.5 ppm and 32.5 ppm in ca. 8:1 ratio) at 20 °C ratio of the two singlets depended on the temperature (ca. 16:1 at 50 °C and ca. 4:1 a °C) (see Supplementary Materials, Figure S1). We think that this Pd complex ado planar geometry and performs cis/trans isomerization; the signal of δ 25.5 ppm wa signed to trans-3 and that of δ 32.5 ppm to cis-3. The thermal cis/trans isomerizatio bis(phosphine)palladium(II) complex is well known [32,33]. In our system, the therm namic parameters ∆H° (1.3 × 10 4 Jmol −1 ) and ∆S° (6.2 × 10 1 JK −1 mol −1 ) of 3 were obta using van't Hoff plots (see Supplementary Materials, Figure S2). These values were lar to those in the previous report [32,33]. It is noteworthy that gram-scale practical reactions were successfully performed: phosphinecarboxamide 1a and phosphinecarbothioamide 2a were isolated in 90% and 99% yields, respectively (Scheme 5). It is noteworthy that gram-scale practical reactions were successfully performed: phosphinecarboxamide 1a and phosphinecarbothioamide 2a were isolated in 90% and 99% yields, respectively (Scheme 5).

Synthesis of Palladium(II) Complexes with Phosphinecarboxamide and Phosphinecarbothioamide
The reaction of Pd(COD)Cl2 with 1a at a 1:2 molar ratio produced the related bis(phosphinecarboxamide)palladium complex [Pd{Ph2PC(O)NHPh-κP}2Cl2] (3) in 97% yield. Complex 3 was obtained even when Pd(COD)Cl2 was treated with 1 equiv. of 1a (NMR yield: 48%) (Scheme 6). The NH proton signal of 1a was not observed in CDCl3 because of the overlapped phenyl protons, but it was observed at δ 7.13 ppm in C6D6 [20]. In the 1 H NMR spectrum of 3 in CDCl3, the NH proton was observed at δ 10.56 ppm. This chemical shift indicates that the NH of 3 is involved in hydrogen bonding. The 31 P NMR spectrum of 3 showed two singlets (δ 25.5 ppm and 32.5 ppm in ca. 8:1 ratio) at 20 °C. The ratio of the two singlets depended on the temperature (ca. 16:1 at 50 °C and ca. 4:1 at −50 °C) (see Supplementary Materials, Figure S1). We think that this Pd complex adopts a planar geometry and performs cis/trans isomerization; the signal of δ 25.5 ppm was assigned to trans-3 and that of δ 32.5 ppm to cis-3. The thermal cis/trans isomerization of bis(phosphine)palladium(II) complex is well known [32,33]. In our system, the thermodynamic parameters ∆H° (1.3 × 10 4 Jmol −1 ) and ∆S° (6.2 × 10 1 JK −1 mol −1 ) of 3 were obtained using van't Hoff plots (see Supplementary Materials, Figure S2). These values were similar to those in the previous report [32,33].

Synthesis of Palladium(II) Complexes with Phosphinecarboxamide and Phosphinecarbothioamide
The reaction of Pd(COD)Cl 2 with 1a at a 1:2 molar ratio produced the related bis(phosphinecarboxamide)palladium complex [Pd{Ph 2 PC(O)NHPh-κP} 2 Cl 2 ] (3) in 97% yield. Complex 3 was obtained even when Pd(COD)Cl 2 was treated with 1 equiv. of 1a (NMR yield: 48%) (Scheme 6). The NH proton signal of 1a was not observed in CDCl 3 because of the overlapped phenyl protons, but it was observed at δ 7.13 ppm in C 6 D 6 [20]. In the 1 H NMR spectrum of 3 in CDCl 3 , the NH proton was observed at δ 10.56 ppm. This chemical shift indicates that the NH of 3 is involved in hydrogen bonding. The 31 P NMR spectrum of 3 showed two singlets (δ 25.5 ppm and 32.5 ppm in ca. 8:1 ratio) at 20 • C. The ratio of the two singlets depended on the temperature (ca. 16:1 at 50 • C and ca. 4:1 at −50 • C) (see Supplementary Materials, Figure S1). We think that this Pd complex adopts a planar geometry and performs cis/trans isomerization; the signal of δ 25.5 ppm was assigned to trans-3 and that of δ 32.5 ppm to cis-3. The thermal cis/trans isomerization of bis(phosphine)palladium(II) complex is well known [32,33]. In our system, the thermodynamic parameters ∆H • (1.3 × 10 4 Jmol −1 ) and ∆S • (6.2 × 10 1 JK −1 mol −1 ) of 3 were obtained using van't Hoff plots (see Supplementary Materials, Figure S2). These values were similar to those in the previous report [32,33]. °C) (see Supplementary Materials, Figure S1). We think that this Pd complex adopts a planar geometry and performs cis/trans isomerization; the signal of δ 25.5 ppm was assigned to trans-3 and that of δ 32.5 ppm to cis-3. The thermal cis/trans isomerization of bis(phosphine)palladium(II) complex is well known [32,33]. In our system, the thermodynamic parameters ∆H° (1.3 × 10 4 Jmol −1 ) and ∆S° (6.2 × 10 1 JK −1 mol −1 ) of 3 were obtained using van't Hoff plots (see Supplementary Materials, Figure S2). These values were similar to those in the previous report [32,33]. Since single crystals were obtained from the reaction solution of Scheme 6, an X-ray structure analysis was performed, and the ORTEP drawing was obtained, as depicted in Figure 3, with the atomic numbering scheme. The complex (trans-3) was confirmed to adopt a typical square-planar configuration; the palladium center had two Cl atoms and two P-coordinated Ph 2 PC(O)NHPh (1a), which were trans-positioned with respect to each other. This molecule had two N-H···Cl intramolecular hydrogen bonds (Cl1···H2n (2.28(4) Å), Cl2···H1n (2.22(6) Å)). The hydrogen bonds in addition to the bulkiness of 1a were considered to be the reason for the trans geometry. This is the first example of a planar complex in which two phosphinecarboxamide ligands are coordinated to the trans position. The Pd-P bond distances (2.3574(9), 2.3707(9) Å) were longer than those of previously reported for cis-[PdCl 2 {(±)-pbap} 2 ] (pbap = 3-phenyl-1,3-dihydrobenzo [1,3]azaphosphol-2-one) (2.2420(7), 2.2282(7) Å) [2]. The sum of the angles around N1 and N2 atoms were ca. 360 • . These angles were similar to those previously reported [31]. This observation indicates that the delocalization of π-electron density between the carbonyl and amide groups was maintained even after complex formation. Since single crystals were obtained from the reaction solution of Scheme 6, an X-ray structure analysis was performed, and the ORTEP drawing was obtained, as depicted in Figure 3, with the atomic numbering scheme. The complex (trans-3) was confirmed to adopt a typical square-planar configuration; the palladium center had two Cl atoms and two P-coordinated Ph2PC(O)NHPh (1a), which were trans-positioned with respect to each other. This molecule had two N-H···Cl intramolecular hydrogen bonds (Cl1···H2n (2.28(4) Å), Cl2···H1n (2.22(6) Å)). The hydrogen bonds in addition to the bulkiness of 1a were considered to be the reason for the trans geometry. This is the first example of a planar complex in which two phosphinecarboxamide ligands are coordinated to the trans position. The Pd-P bond distances (2.3574(9), 2.3707(9) Å) were longer than those of previously reported for cis-[PdCl2{(±)-pbap}2] (pbap = 3-phenyl-1,3-dihydrobenzo [1,3]azaphosphol-2-one) (2.2420(7), 2.2282(7) Å) [2]. The sum of the angles around N1 and N2 atoms were ca. 360°. These angles were similar to those previously reported [31]. This observation indicates that the delocalization of π-electron density between the carbonyl and amide groups was maintained even after complex formation.   Figure 4. Coupled two doublets with 2 J(PP) = 453.4 Hz were observed at δ -54.2 ppm, 37.8 ppm and those with 2 J(PP) = 26.2 Hz were observed at δ -41.9 ppm, 37.3 ppm at 20 • C. We assigned the signals with the largest coupling constant to the trans complex and those with the smallest coupling constant to the cis complex. The ratio of the cis complex increased when the measurement temperature was lowered (ca. 1.8:1 at 20 • C, 2.6:1 at −10 • C, and ca. 4.1:1 at −40 • C), and this ratio changed reversibly with the temperature. This behavior has not been reported in previous complexes having phosphinecarbothioamide. Thermodynamic parameters ∆H • (8.6 × 10 3 Jmol −1 ) and ∆S • (2.5 × 10 1 JK −1 mol −1 ) of 5 were obtained using van't Hoff plots and were similar to the values found for 3 (see Supplementary Materials, Figure S3).   The structures of 4 and 5 were studied via an X-ray crystal structure analysis. Two independent molecules of 4 crystallized in the unit cell. As these were basically the same, only one molecule (Pd1) is shown in Figure 5a). Complex 4 had a typical square-planar environment; palladium had two Cl atoms situated mutually in cis position and P,S-coordinated bidentate ligand 2a. The bond lengths of Pd-P (2.2064(7), 2.1887(7) Å), Pd-S (2.2954(8), 2.2928(8) Å), and Pd-Cl (2.3299(7), 2.3986(7) Å) were similar to those of the previously reported [PdCl2{Ph2PC(S)NMe2-κP,S}] complex (2.209(1) Å for Pd-P bond, 2.290(1) Å for Pd-S bond, and 2.329(1), 2.376(1) Å for Pd-Cl bond) [11]. Intermolecular hydrogen bonds between the H1n atoms for the Pd1 molecule and the Cl4 atom for the Pd2 molecule were observed (H1n···Cl4 (2.28(2) Å)). Compound 5 consisted of a cationic palladium complex and a Clcounter anion (Figure 5b)). In the cationic Pd complex, the Pd atom is coordinated to the phosphorus atom of 2a as a monodentate ligand, a bidentate P,S-bonded 2a, and a Cl atom, acquiring a square-planar configuration. The two phosphorus atoms were cis with respect to each other. This is the first example of a transition-metal complex with both monodentate and bidentate phosphinecarbothioamide ligands. Complex 5 also showed inter-molecular hydrogen bonds (2.18(3) Å for H1n···Cl2 and 2.29(3) Å for H2n···Cl2) (Figure 6) [25].   The structures of 4 and 5 were studied via an X-ray crystal structure analysis. Two independent molecules of 4 crystallized in the unit cell. As these were basically the same, only one molecule (Pd1) is shown in Figure 5a). Complex 4 had a typical square-planar environment; palladium had two Cl atoms situated mutually in cis position and P,S-coordinated bidentate ligand 2a. The bond lengths of Pd-P (2.2064(7), 2.1887(7) Å), Pd-S (2.2954(8), 2.2928(8) Å), and Pd-Cl (2.3299(7), 2.3986(7) Å) were similar to those of the previously reported [PdCl2{Ph2PC(S)NMe2-κP,S}] complex (2.209(1) Å for Pd-P bond, 2.290(1) Å for Pd-S bond, and 2.329(1), 2.376(1) Å for Pd-Cl bond) [11]. Intermolecular hydrogen bonds between the H1n atoms for the Pd1 molecule and the Cl4 atom for the Pd2 molecule were observed (H1n···Cl4 (2.28(2) Å)). Compound 5 consisted of a cationic palladium complex and a Clcounter anion (Figure 5b)). In the cationic Pd complex, the Pd atom is coordinated to the phosphorus atom of 2a as a monodentate ligand, a bidentate P,S-bonded 2a, and a Cl atom, acquiring a square-planar configuration. The two phosphorus atoms were cis with respect to each other. This is the first example of a transition-metal complex with both monodentate and bidentate phosphinecarbothioamide ligands. Complex 5 also showed inter-molecular hydrogen bonds (2.18(3) Å for H1n···Cl2 and 2.29(3) Å for H2n···Cl2) (Figure 6) [25]. The structures of 4 and 5 were studied via an X-ray crystal structure analysis. Two independent molecules of 4 crystallized in the unit cell. As these were basically the same, only one molecule (Pd1) is shown in Figure 5a). Complex 4 had a typical squareplanar environment; palladium had two Cl atoms situated mutually in cis position and P,S-coordinated bidentate ligand 2a. The bond lengths of Pd-P (2.2064(7), 2.1887(7) Å), Pd-S (2.2954(8), 2.2928(8) Å), and Pd-Cl (2.3299(7), 2.3986(7) Å) were similar to those of the previously reported [PdCl 2 {Ph 2 PC(S)NMe 2 -κP,S}] complex (2.209(1) Å for Pd-P bond, 2.290(1) Å for Pd-S bond, and 2.329(1), 2.376(1) Å for Pd-Cl bond) [11]. Intermolecular hydrogen bonds between the H1n atoms for the Pd1 molecule and the Cl4 atom for the Pd2 molecule were observed (H1n···Cl4 (2.28(2) Å)). Compound 5 consisted of a cationic palladium complex and a Clcounter anion (Figure 5b)). In the cationic Pd complex, the Pd atom is coordinated to the phosphorus atom of 2a as a monodentate ligand, a bidentate P,S-bonded 2a, and a Cl atom, acquiring a square-planar configuration. The two phosphorus atoms were cis with respect to each other. This is the first example of a

General Considerations
The synthesis of phosphinecarboxamide and phosphinecarbothioamide was carried out using standard Schlenk techniques in a dry nitrogen atmosphere. The synthesis of complexes 3-5 was carried out in air. Iron complex CpFe(CO)2(Me) [34] and palladium complex PdCl2(COD) [35] were prepared according to the literature method. The other chemicals were commercially available. Phenylphosphine PhPH2 (10% hexane solution) was used after distillation (however, a little amount of hexane could not be removed). Solvents were purified employing a two-column solid-state purification system or were distilled from appropriate drying agents under N2. Spectroscopic data of the products obtained in this work, 1a-1g, 2a-2d, 2f, and 2k, agreed with those in the literatures [16,17]. Spectroscopic data of the products 1i-1s were preliminarily reported in [27]. NMR spectra ( 1 H, 13 C, and 31 P) were recorded at ambient temperature with a JNM ECS-400 spectrometer. 1 H and 13 C NMR data were referred to residual peaks of solvent as an internal standard. Peak positions of 31 P NMR spectra were referenced to external 85% H3 PO4 (δ = 140 ppm). Elemental analysis data were obtained with a Perkin-Elmer 2400 CHN elemental analyzer.

General Considerations
The synthesis of phosphinecarboxamide and phosphinecarbothioamide was carried out using standard Schlenk techniques in a dry nitrogen atmosphere. The synthesis of complexes 3-5 was carried out in air. Iron complex CpFe(CO)2(Me) [34] and palladium complex PdCl2(COD) [35] were prepared according to the literature method. The other chemicals were commercially available. Phenylphosphine PhPH2 (10% hexane solution) was used after distillation (however, a little amount of hexane could not be removed). Solvents were purified employing a two-column solid-state purification system or were distilled from appropriate drying agents under N2. Spectroscopic data of the products obtained in this work, 1a-1g, 2a-2d, 2f, and 2k, agreed with those in the literatures [16,17]. Spectroscopic data of the products 1i-1s were preliminarily reported in [27]. NMR spectra ( 1 H, 13 C, and 31 P) were recorded at ambient temperature with a JNM ECS-400 spectrometer. 1 H and 13 C NMR data were referred to residual peaks of solvent as an internal standard. Peak positions of 31 P NMR spectra were referenced to external 85% H3 PO4 (δ = 140 ppm). Elemental analysis data were obtained with a Perkin-Elmer 2400 CHN elemental analyzer.

General Considerations
The synthesis of phosphinecarboxamide and phosphinecarbothioamide was carried out using standard Schlenk techniques in a dry nitrogen atmosphere. The synthesis of complexes 3-5 was carried out in air. Iron complex CpFe(CO) 2 (Me) [34] and palladium complex PdCl 2 (COD) [35] were prepared according to the literature method. The other chemicals were commercially available. Phenylphosphine PhPH 2 (10% hexane solution) was used after distillation (however, a little amount of hexane could not be removed). Solvents were purified employing a two-column solid-state purification system or were distilled from appropriate drying agents under N 2 . Spectroscopic data of the products obtained in this work, 1a-1g, 2a-2d, 2f, and 2k, agreed with those in the literatures [16,17]. Spectroscopic data of the products 1i-1s were preliminarily reported in [27]. NMR spectra ( 1 H, 13 C, and 31 P) were recorded at ambient temperature with a JNM ECS-400 spectrometer. 1 H and 13 C NMR data were referred to residual peaks of solvent as an internal standard. Peak positions of 31 P NMR spectra were referenced to external 85% H 3 PO 4 (δ = 140 ppm). Elemental analysis data were obtained with a Perkin-Elmer 2400 CHN elemental analyzer.

General Procedure for Synthesis of Phosphinecarboxamide and Phosphinecarbothioamide
The mixture of the phosphine compound (0.7 mmol) and isocyanate/isothiocyanate (0.7 mmol) was stirred at room temperature in a Schlenk tube. After the reaction was complete, all volatile materials were removed under reduced pressure. The residual powder was dried in vacuo to give the corresponding phosphinecarboxamide/phosphinecarbothioamide.  62 (m, 4H). The peak of NH overlapped with another peak. 13 C NMR (100.4 MHz, CDCl 3 , ppm) δ 32.2, 58.0, 129.6 (d, 3 J C-P = 8.3 Hz,

Crystallography
Crystallographic data are summarized in Table 1. The single crystals of 1i, 2c, and 3-5 were obtained using the slow diffusion method (CH 2 Cl 2 /hexane for 3 and 5; acetone/ether for 4). Diffraction-intensity data were collected with Rigaku AFC11 with a Saturn 724 + CCD diffractometer (200(2) K for 1i, 2c; 110(2) K for 3-5), and semiempirical multi-scan absorption correction [36] was performed. The structures were solved using SIR97 [37] via subsequent difference Fourier synthesis, and refined with full matrix least-squares procedures on F 2 . All non-hydrogen atoms were refined with anisotropic displacement coefficients. NH protons were determined via difference Fourier synthesis and refined isotropically. Hydrogen atoms (except for NH protons) were treated as idealized contributions and refined in a rigid group model. All software and sources of scattering factors were contained in the SHELXL-2018/3 [38] program package. The Cambridge Crystallographic Data Centre (CCDC) deposition numbers of 1i, 2c, and 3-5 are included in Table 1.

Conclusions
We achieved the catalyst-free synthesis of phosphinecarboxamide and phosphinecarbothioamide through the hydrophosphination of isocyanates and isothiocyanates. The important point in our system was to carry out the reaction without using a solvent (neat). This system showed the characteristics of easy handling, high yield, short reaction time, and good functional-group tolerance for the functionalized isocyanates and isothiocyanates. For the requirement of no catalyst nor solvent, the gram-scale synthesis of these compounds was also achieved. In addition, we synthesized and characterized palladium complexes with phosphinecarboxamide and phosphinecarbothioamide as ligands. In the reaction of Pd(COD)Cl 2 with 1a, [Pd{Ph 2 PC(O)NHPh-κP} 2 Cl 2 ] (3) was obtained regardless of whether 1a was used in 1 or 2 equivalents. In contrast, in the reaction of Pd(COD)Cl 2 with 2a, [PdCl 2 {Ph 2 PC(S)NHPh-κP,S}] (4) and [PdCl{Ph 2 PC(S)NHPh-κP,S}{Ph 2 PC(S)NHPh-κP}]Cl (5) were selectively obtained when 1 eq. and 2 eq. of 2a were used, respectively. It was revealed that 3 and 5 having intra-and/or inter-molecular hydrogen bonds showed thermal cis/trans isomerization.