Synthesis, Structure, and Reactivity of Molybdenum– and Tungsten–Indane Complexes with Tris(pyrazolyl)borate Ligand

The reaction of molybdenum complexes with a tris(pyrazolyl)borate ligand (Et4N[TpMo(CO)3] and Et4N[Tp*Mo(CO)3] (Tp = hydridotris(pyrazolyl)borate, Tp* = hydridotris(3,5-dimethylpyrazolyl)borate)) and InBr3 at a 1:1 molar ratio afforded molybdenum–indane complexes (Et4N[TpMo(CO)3(InBr3)] 1 and Et4N[Tp*Mo(CO)3(InBr3)] 2). In addition, tungsten–indane complexes, Et4N[TpW(CO)3(InBr3)] 3 and Et4N[Tp*W(CO)3(InBr3)] 4, were obtained by the reaction of corresponding tungsten complexes. Complex 4 reacted with H2O to form the hydrido complex Tp*W(CO)3H, in which the W–In bond was cleaved. On the other hand, 4 reacted with three equiv. of AgNO3 to form Et4N[Tp*W(CO)3{In(ONO2)}] 5, in which three substituents on the In were exchanged while retaining the W–In dative bond. Complexes 1–5 were fully characterized using NMR measurements and elemental analyses, and the structures of 1–5 and Et4N[Tp*W(CO)3] were determined via X-ray crystallography. These are the first examples of mononuclear molybdenum– and tungsten–indane complexes with Mo–In and W–In dative bonds.


Introduction
Common compounds with Group 13 elements as the central atom are designated as EX 3 (E = B, Al, Ga, In, Tl) and are known as electron-deficient compounds.Therefore, for bonds formed between a Group 13 element and a transition metal (TM), a TM→E dative bond can be considered in addition to a TM-E covalent bond (for example, TM-EX 2 ) (Figure 1 shows an example in which E = In) [1][2][3][4][5][6][7].A normal bond between a TM and a ligand is formed by donating two electrons from the ligand to the TM.On the other hand, the TM→E dative bond is a bond created when two electrons are donated from the TM to E. This is called a Z-type interaction and it is a bond that is unique to Group 13 element compounds [8][9][10][11][12][13][14][15].Although many compounds with TM-B bonds have been reported, there are not many reported examples of compounds with TM-In bonds, and among them, there are few examples of compounds with W-In bonds.[17].A thiocyanate indylene complex [{CpW(CO) 3 } 2 In (NCS)] was synthesized by Norman in 1995 [18].Reger and Rheingold used a bulky In compound, Tp*InCl 2 •THF, to prepare the W-In complex [W(CO) 5 (InTp*)] [19].A trihaloin- date W complex [Ph 4 P] 2 [(CO) 5 W-In I Cl 3 ] containing indium in a formal oxidation state of +I was obtained by the reaction of a reactive In I complex [(CO) 5 WInCl•THF] with an excess amount of [Ph 4 P]Cl [20].We have also reported the pyridine-stabilized indylene complex [{Cp(CO) 3 W} 2 InCl(py)] [21].It is reasonable that the W-In bond in the complexes listed above is a covalent bond.However, it is also possible to interpret it as a W→In dative bond.
In fact, the nature of the W-In bond is not clearly stated in these papers.Figure 2 shows the complexes with W-In bonds that have been reported to date.In 1988, Norman et al. reported [In{CpW(CO)3}3], Na[{CpW(CO)3}2InCl2], and Na[{CpW(CO)3}InCl3] [16] as the first complexes with W-In bond(s).A crystal structure was reported in 2002 for [In{CpW(CO)3}3] by Scheer [17].A thiocyanate indylene complex [{CpW(CO)3}2In(NCS)] was synthesized by Norman in 1995 [18].Reger and Rheingold used a bulky In compound, Tp*InCl2•THF, to prepare the W-In complex [W(CO)5(InTp*)] [19].A trihaloindate W complex [Ph4P]2[(CO)5W-In I Cl3] containing indium in a formal oxidation state of +I was obtained by the reaction of a reactive In I complex [(CO)5WInCl•THF] with an excess amount of [Ph4P]Cl [20].We have also reported the pyridine-stabilized indylene complex [{Cp(CO)3W}2InCl(py)] [21].It is reasonable that the W-In bond in the complexes listed above is a covalent bond.However, it is also possible to interpret it as a W→In dative bond.In fact, the nature of the W-In bond is not clearly stated in these papers.Although there are no examples of structural analyses of complexes with clear W→In dative bonds, several examples of structural analyses of complexes with TM→In dative bonds have been reported for other transition metal complexes: one example for Pt [15], Pd [22], and Ru [23]; two examples for Co [24,25]; and three examples for Ni [26][27][28] and Rh [29][30][31].
In general, polynuclear complexes have several possibilities for how to allocate bonding electrons.The same is true for complexes with TM-In bonds.In other words, when a complex with a TM-In bond has a multinuclear structure, it becomes difficult to determine whether the TM-In bond is a dative bond or a covalent bond.In this study, we focus on complexes with clear TM-In dative bonds.To obtain a complex with a clear TM-In dative bond, it is necessary to prepare a mononuclear complex.Tp (hydridotris(pyrazolyl)borate) and Tp* (hydridotris(3,5-dimethylpyrazolyl)borate) have been widely used as six electron donor anionic ligands, like Cp (cyclopentadienyl) and Cp* (pentamethylcyclopentadienyl) ligands.However, since Tp and Tp* are bulkier than Cp and Cp*, complexes with Tp or Tp* as a ligand are less likely to form polynuclear complexes.Here, we selected Mo and W complexes with a Tp or Tp* ligand and examined their reaction with InBr3.Although there are no examples of structural analyses of complexes with clear W→In dative bonds, several examples of structural analyses of complexes with TM→In dative bonds have been reported for other transition metal complexes: one example for Pt [15], Pd [22], and Ru [23]; two examples for Co [24,25]; and three examples for Ni [26][27][28] and Rh [29][30][31].

Synthesis and
In general, polynuclear complexes have several possibilities for how to allocate bonding electrons.The same is true for complexes with TM-In bonds.In other words, when a complex with a TM-In bond has a multinuclear structure, it becomes difficult to determine whether the TM-In bond is a dative bond or a covalent bond.In this study, we focus on complexes with clear TM-In dative bonds.To obtain a complex with a clear TM-In dative bond, it is necessary to prepare a mononuclear complex.Tp (hydridotris(pyrazolyl)borate) and Tp* (hydridotris(3,5-dimethylpyrazolyl)borate) have been widely used as six electron donor anionic ligands, like Cp (cyclopentadienyl) and Cp* (pentamethylcyclopentadienyl) ligands.However, since Tp and Tp* are bulkier than Cp and Cp*, complexes with Tp or Tp* as a ligand are less likely to form polynuclear complexes.Here, we selected Mo and W complexes with a Tp or Tp* ligand and examined their reaction with InBr 3 .Single crystals of 1 and 2 were grown from a hot acetonitrile solution.The molecular structures of 1 and 2 were determined via X-ray crystallography.The ethyl groups of cationic parts were disordered, so the cationic parts were omitted for simplicity, and an ORTEP drawing of the anionic parts is depicted in Figure 3 with an atomic numbering scheme.Selected bond lengths (Å) and angles ( • ) are summarized in Table 1.To estimate the bond interaction of TM-E, the r value (the ratio of The M-E bond length to the sum of the TM and E covalent radii) has been used [25,[33][34][35].Thus, the r values are also listed in Table 1.Both 1 and 2 are seven-coordinated complexes.The Mo(0) center of 1 has a four-legged piano stool geometry, with one tridentate Tp ligand, three terminal CO ligands, and one InBr 3 ligand.In contrast, the Mo(0) center of 2 is coordinated by one tridentate Tp* ligand, three terminal CO ligands, and one InBr 3 ligand in a 3:3:1 face-capped octahedral arrangement.The capped octahedral structure is similar to the previously reported tin analog [Tp*Mo(CO) 3 (SnPh 3 )] [36].The structural difference between the Tp and Tp* complexes is considered to come from the steric repulsion between the methyl groups on the Tp* ligand and the InBr 3 ligand.The lengths of the Mo-In bond (2.8575(7) Å for 1, 2.8117(8) Å for 2) and In-Br 3 bond (2.5476(8) Å, 2.5630(9) Å, and 2.5486(9) Å for 1, 2.5579(9) Å, 2.5408(10) Å, and 2.5467(10) Å for 2) are shorter than the sum of the Mo and In covalent radii (2.96 Å) and that of the In and Br covalent radii (2.68 Å), respectively [36]  Single crystals of 1 and 2 were grown from a hot acetonitrile solution.The molecula structures of 1 and 2 were determined via X-ray crystallography.The ethyl groups of cat ionic parts were disordered, so the cationic parts were omi ed for simplicity, and an OR TEP drawing of the anionic parts is depicted in Figure 3 with an atomic numberin scheme.Selected bond lengths (Å) and angles (°) are summarized in Table 1.To estimat the bond interaction of TM-E, the r value (the ratio of The M-E bond length to the sum o the TM and E covalent radii) has been used [25,[33][34][35].Thus, the r values are also listed in Table 1.around the metal center and the tendencies of the W-N bond lengths and the W-CO bond lengths of 3 and 4 are similar to those of 1 and 2, respectively.Although the Br-In-Br angles for 3 (99.87(2)° to 105.50(2)°) and 4 (102.63(4)° to 103.41(4)°) are similar to the corresponding 1 and 2, the r values of 3 (0.94) and 4 (0.93) are smaller than those of 1 (0.97) and 2 (0.95), suggesting that the M→In interaction is larger for W than Mo and for the Tp* complex than for the Tp complex.In other words, it is inferred that 4 has the strongest TM→In III dative bond among 1-4.a Ratio of the W-In bond length to the sum of W and In covalent radii [35].

Reactivity of Tungsten-Indane Complexes with Tp* Ligand
For indane complexes 1-4, which were prepared and isolated in this work, it was shown that the W-In dative bond of 4 was the strongest.Therefore, in order to investigate the possibility of this bond cleavage, the reaction of 4 with a Lewis base was conducted.In addition, in order to investigate whether the In-Br bond can be cleaved while retaining the W-In bond, the reactions of 4 with some Ag salts were examined.a Ratio of the W-In bond length to the sum of W and In covalent radii [35].

Reactivity of Tungsten-Indane Complexes with Tp* Ligand
For indane complexes 1-4, which were prepared and isolated in this work, it was shown that the W-In dative bond of 4 was the strongest.Therefore, in order to investigate the possibility of this bond cleavage, the reaction of 4 with a Lewis base was conducted.In addition, in order to investigate whether the In-Br bond can be cleaved while retaining the W-In bond, the reactions of 4 with some Ag salts were examined.
Water was selected as a Lewis base to react with 4. Complex 4 was added to water and stirred at room temperature for 2 h, but no change occurred.However, when acetonitrile was added to this, 4 was converted into the hydrido complex (Tp*W(CO) 3 H) with a 64% yield (Scheme 3).Templeton et al. reported that the hydrido complex (Tp*W (CO) 3 H) was obtained in the reaction of Et 4 N[Tp*W(CO) 3 ] with HCl [39].Therefore, our observation shown in Scheme 3 can be explained in the following two ways.(i) InBr 3 (a Lewis acid) forms a stronger bond with the Tp*W(CO) 3 fragment than with H 2 O, so InBr 3 does not dissociate from the W fragment in water, leading to no reaction.On the other hand, InBr 3 forms a stronger bond with MeCN than Tp*W(CO) 3 , so when MeCN is added,  4) is poorly soluble in water, no solid-liquid reaction occurred.To investigate the possibility of (ii), we synthesized K[Tp*W(CO) 3 (InBr 3 )], which shows better solubility in water than the NEt 4 salt, and stirred this complex in water, confirming the formation of Tp*W(CO) 3 H.Therefore, the reason why 4 did not react with water is because of its extremely low solubility in water.The reactions mentioned above revealed that InBr 3 inherently prefers to interact with H 2 O and MeCN rather than the Tp*W(CO) 3 fragment.
this complex in water, confirming the formation of Tp*W(CO)3H.Therefore, why 4 did not react with water is because of its extremely low solubility in reactions mentioned above revealed that InBr3 inherently prefers to interact wi MeCN rather than the Tp*W(CO)3 fragment.Since Ag salts are widely used as halogen abstraction reagents, reaction some Ag salts were examined.Complex 4 and three equiv. of Ag salt were stirre at room temperature for 3 h in the dark.When AgClO3, AgOAc, AgOTf, and A used, complicated reactions occurred, and the product could not be identified action of 4 with AgNO3, all Br atoms on the indium were replaced by three N without breaking the W-In dative bond, and the corresponding Et4N[Tp*W(CO)3{In(ONO2)}] (5) was obtained with a 90% yield (Scheme 4).T seems to be applicable to the synthesis of various transition metal-indane com Complex 5 was characterized via a single-crystal X-ray diffraction analys onic part was omi ed for simplicity, an ORTEP drawing of the anionic part i Figure 5, and selected bond lengths, angles, and r values are summarized in T geometry about the W atom is a capped octahedral with the indium located position and a structure that is analogous to that of 4. Since In has a high elect ing ability, when it has an NO3 substituent, it often adopts an η 2 -type bonding accepts three electrons rather than adopting an η 1 -type bonding mode and acc electron.The crystal structure of an In compound with three NO3 grou dmbipy)(η 2 -NO3)2(η 1 -NO3)(H2O)] (4,4′-dmbipy = 4,4′-dimethyl-2,2′-bipyridin ported, in which two NO3 are bonded in the η 2 -type bonding mode and one NO Since Ag salts are widely used as halogen abstraction reagents, reactions of 4 with some Ag salts were examined.Complex 4 and three equiv. of Ag salt were stirred in MeCN at room temperature for 3 h in the dark.When AgClO 3 , AgOAc, AgOTf, and AgBF 4 were used, complicated reactions occurred, and the product could not be identified.In the reaction of 4 with AgNO 3 , all Br atoms on the indium were replaced by three NO 3 groups without breaking the W-In dative bond, and the corresponding complex Et 4 N[Tp*W(CO) 3 {In(ONO 2 )}] ( 5) was obtained with a 90% yield (Scheme 4).This method seems to be applicable to the synthesis of various transition metal-indane complexes.
MeCN rather than the Tp*W(CO)3 fragment.Since Ag salts are widely used as halogen abstraction reagents, reactions of 4 some Ag salts were examined.Complex 4 and three equiv. of Ag salt were stirred in M at room temperature for 3 h in the dark.When AgClO3, AgOAc, AgOTf, and AgBF4 used, complicated reactions occurred, and the product could not be identified.In t action of 4 with AgNO3, all Br atoms on the indium were replaced by three NO3 g without breaking the W-In dative bond, and the corresponding com Et4N[Tp*W(CO)3{In(ONO2)}] ( 5) was obtained with a 90% yield (Scheme 4).This m seems to be applicable to the synthesis of various transition metal-indane complexe Complex 5 was characterized via a single-crystal X-ray diffraction analysis.Th onic part was omi ed for simplicity, an ORTEP drawing of the anionic part is sho Figure 5, and selected bond lengths, angles, and r values are summarized in Table 3 geometry about the W atom is a capped octahedral with the indium located at an position and a structure that is analogous to that of 4. Since In has a high electron-a ing ability, when it has an NO3 substituent, it often adopts an η 2 -type bonding mod accepts three electrons rather than adopting an η 1 -type bonding mode and acceptin electron.The crystal structure of an In compound with three NO3 groups [In dmbipy)(η 2 -NO3)2(η 1 -NO3)(H2O)] (4,4′-dmbipy = 4,4′-dimethyl-2,2′-bipyridine) wa ported, in which two NO3 are bonded in the η 2 -type bonding mode and one NO3 is bo Complex 5 was characterized via a single-crystal X-ray diffraction analysis.The cationic part was omitted for simplicity, an ORTEP drawing of the anionic part is shown in Figure 5, and selected bond lengths, angles, and r values are summarized in Table 3.The geometry about the W atom is a capped octahedral with the indium located at an axial position and a structure that is analogous to that of 4. Since In has a high electronaccepting ability, when it has an NO 3 substituent, it often adopts an η 2 -type bonding mode and accepts three electrons rather than adopting an η 1 -type bonding mode and accepting one electron.The crystal structure of an In compound with three NO groups, one In-O length (2.176(2)-2.183(2)Å) is significantly shorter than the other two In-O lengths (2.623-2.744Å).Thus, all of the three NO3 groups in the In form η 1 -type bonds (only one O atom makes a bond with In), and none of them form η 2 -type bonds (base-stabilized type).This can be considered to be a reflection of the Tp*W(CO)3 fragment donating a sufficient electron density to the In.Its r value of 5 (0.92) is the smallest among those of the complexes reported in this work.a Ratio of the W-In bond length to the sum of W and In covalent radii [35].

General Considerations
All manipulations were carried out using standard Schlenk techniques under a dry nitrogen atmosphere.Molybdenum and tungsten complexes with Tp or Tp* ligands (Et4N[TpM(CO)3] and Et4N[Tp*M(CO)3] (M = Mo, W)) were prepared according to a method from the literature [32].The other chemicals were commercially available.Solvents were purified employing a two-column solid-state purification system or were distilled from appropriate drying agents under N2.NMR spectra ( 1 H and 13 C) were recorded at ambient temperature on a JEOL JNM AL-400 spectrometer. 1 H and 13 C NMR data  a Ratio of the W-In bond length to the sum of W and In covalent radii [35].

General Considerations
All manipulations were carried out using standard Schlenk techniques under a dry nitrogen atmosphere.Molybdenum and tungsten complexes with Tp or Tp* ligands (Et 4 N[TpM(CO) 3 ] and Et 4 N[Tp*M(CO) 3 ] (M = Mo, W)) were prepared according to a method from the literature [32].The other chemicals were commercially available.Solvents were purified employing a two-column solid-state purification system or were distilled from appropriate drying agents under N 2 .NMR spectra ( 1 H and 13 were recorded at ambient temperature on a JEOL JNM AL-400 spectrometer. 1 H and 13 C NMR data referred to residual peaks of solvent as an internal standard.Elemental analysis data were obtained with a Perkin-Elmer 2400 CHN elemental analyzer.

Crystallography
Crystallographic data and details of structure refinement parameters are summarized in Tables 4 and 5.The single crystals 1-4 were grown from a hot acetonitrile solution.The single crystals of 5 were obtained using the slow diffusion method (acetonitrile/ether). Diffraction intensity data were collected with a Rigaku AFC11 with Saturn 724+ CCD diffractometer, and a semiempirical multi-scan absorption correction [41] was performed.The structures were solved using SIR97 [42] by subsequent difference Fourier syntheses and refined by full-matrix least-squares procedures on F 2 .All non-hydrogen atoms were refined with anisotropic displacement coefficients.Hydrogen atoms were treated as idealized contributions and refined in rigid group model.All software and sources of scattering factors are contained in the SHELXL-97 [43] and the SHELXL-2018/3 [44] program package.The Cambridge Crystallographic Data Centre (CCDC) deposition numbers of 1-5 and Et 4 N[Tp*W(CO) 3 ] are included in Tables 4 and 5.

Conclusions
We are able to show the first examples of the synthesis and structure of molybdenumand tungsten-indane complexes (1)(2)(3)(4).These complexes were obtained by the reactions of corresponding molybdenum and tungsten complexes with Tp or Tp* ligands with one

Figure 2
Figure 2 shows the complexes with W-In bonds that have been reported to date.In 1988, Norman et al. reported [In{CpW(CO) 3 } 3 ], Na[{CpW(CO) 3 } 2 InCl 2 ], and Na[{CpW(CO) 3 } InCl 3 ] [16] as the first complexes with W-In bond(s).A crystal structure was reported in 2002 for [In{CpW(CO) 3 } 3 ] by Scheer [17].A thiocyanate indylene complex [{CpW(CO) 3 } 2 In (NCS)] was synthesized by Norman in 1995 [18].Reger and Rheingold used a bulky In compound, Tp*InCl 2 •THF, to prepare the W-In complex [W(CO) 5 (InTp*)] [19].A trihaloin- date W complex [Ph 4 P] 2 [(CO) 5 W-In I Cl 3 ] containing indium in a formal oxidation state of +I was obtained by the reaction of a reactive In I complex [(CO) 5 WInCl•THF] with an excess amount of [Ph 4 P]Cl[20].We have also reported the pyridine-stabilized indylene complex [{Cp(CO) 3 W} 2 InCl(py)][21].It is reasonable that the W-In bond in the complexes listed above is a covalent bond.However, it is also possible to interpret it as a W→In dative bond.In fact, the nature of the W-In bond is not clearly stated in these papers.
. The indium atoms of 1 and 2 have a pseudo-tetrahedral geometry.The Br-In-Br angles are 100.25(2)• to 105.87(3) • for 1 and 102.44(3) • to 103.85(4) • for 2. To the best of our knowledge, 1 and 2 are the first Mo-indane complexes to be prepared, isolated, and analyzed via their X-ray structure.The Mo-In bond length of 2 is shorter than that of 1, showing that a greater electron density is donated from Mo to In in 2 than in 1.The Mo-N bond lengths (an avg. of 2.251 Å for 1 vs. avg.2.265 Å for Et 4 N[TpMo(CO) 3 ] [37], and an 2.238 Å of for 2 vs. avg.2.263 Å for Et 4 N[Tp*Mo(CO) 3 ] [38]) do not change much before and after the coordination of the InBr 3 ligand to the Mo center.In contrast, a comparison of the Mo-CO bond lengths led to an interesting finding: there was an avg. of 1.977 Å for 1 vs. avg.1.925 Å for Et 4 N[TpMo(CO) 3 ] [37], and an avg. of 1.981 Å for 2 vs. avg.1.941 Å for Et 4 N[Tp*Mo(CO) 3 ] [38].In both cases of 1 and 2, the coordination of InBr 3 to Mo lengthens the Mo-CO bonds.It is thought that the electron density of Mo decreases due to the coordination of the InBr 3 ligand, weakening the π back-donation to the carbonyl ligands.

2. 2 .
Synthesis and Structure of Tungsten-Indane Complexes with Tris(pyrazolyl)borate Ligand Tungsten-indane complexes 3 and 4 were obtained with 92% and 90% yields by the reactions of Et 4 N[TpW(CO) 3 ] [32] and Et 4 N[Tp*W(CO) 3 ] [32] with one equiv.of InBr 3 , respectively (Scheme 2).The 1 H NMR signals due to the Tp ligand of 3 were observed at δ 6.34, 7.84, and 8.21 in CD 3 CN and those due to the Tp* ligand of 4 were observed at δ 2.35, 2.42, and 5.97 in CD 3 CN.They were slightly downfield compared to the signals of the corresponding Mo-indane complexes (1 and 2). r

Scheme 2 .
Scheme 2. Reactions of Et4N[TpW(CO)3] and Et4N[Tp*W(CO)3] with 1 equiv. of InBr3.Scheme 2. Reactions of Et 4 N[TpW(CO) 3 ] and Et 4 N[Tp*W(CO) 3 ] with 1 equiv. of InBr 3 .Single crystals of 3, 4, and Et 4 N[Tp*W(CO) 3 ] were obtained using the same methods as 1 and 2, and X-ray crystal analyses were conducted.The ethyl groups of the cationic parts were disordered, so the cationic parts were omitted for simplicity, and ORTEP drawings of the anionic parts are shown in Figure4.The most characteristic parameters are summarized in Table2.To the best of our knowledge, 3 and 4 are the first examples of the synthesis and structure of a tungsten-indane complex.The coordination environment around the metal center and the tendencies of the W-N bond lengths and the W-CO bond lengths of 3 and 4 are similar to those of 1 and 2, respectively.Although the Br-In-Br angles for 3 (99.87(2)• to 105.50(2) • ) and 4 (102.63(4)• to 103.41(4) • ) are similar to the corresponding 1 and 2, the r values of 3 (0.94) and 4 (0.93) are smaller than those of 1 (0.97) and 2 (0.95), suggesting that the M→In interaction is larger for W than Mo and for the Tp* complex than for the Tp complex.In other words, it is inferred that 4 has the strongest TM→In III dative bond among 1-4.
[35]tio of the Mo-In bond length to the sum of Mo and In covalent radii[35].
[35]o of the Mo-In bond length to the sum of Mo and In covalent radii[35]. a

Table 5 .
Crystallographic data and details of structure refinement parameters of 3-5.