Palladium(II) and Platinum(II) Mononuclear Complexes and Tendency to Undergo Dehydrogenation of the Multiple N-Donor Ligand Di-(2-pyridyl)dihydropyrazine

Ligand Di-(2-pyridyl)dihydropyrazine

Article

Palladium(II) and Platinum(II) Mononuclear Complexes and

Tendency to Undergo Dehydrogenation of the Multiple N‑Donor

Edoardo Raciti, Maria Pia Donzello,* Elisa Viola, Mauro Bassetti,* Ida Pettiti, Noemi Bellucci,

Corrado Rizzoli,* and Claudio Ercolani *

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c00699

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sı Supporting Information

ABSTRACT: The already known di(2-pyridyl)dihydropyrazine

dhdpp) was prepared and isolated also in the form of a bis-

(

hydrated species, i.e., dhdpp·2H O. As established by X-ray work, a

small amount of single crystals of di(2-pyridyl)-pyrazine (dpp) was

also obtained from the mother liquors, this testifying the possibility

of a dehydrogenation process dhdpp → dpp in the absence of a

catalyst. Using dhdpp as a ligand, mononuclear metal derivatives of

formula [(dhdpp)MCl ]·xH O (M = Pd , Pt ) were obtained as

stable-to-air solids, studied by X-ray powder, IR, UV−visible, and

H NMR spectra, and proved to exhibit a N MCl coordination

site involving one pyridine and one pyrazine N atom (“py-pyz”

coordination). An interesting relationship has been established in

terms of the observed types of coordination with the analogs of di(2-pyridyl)-pyrazine (dpp) formulated as [(dpp)MCl ]·3H O,

proved also by H NMR spectra to exhibit the “py-pyz” mode of coordination. Attempts to isolate from the reaction of dhdpp with

Pd(OAc) the corresponding mononuclear derivatives were shown to lead, as deﬁnitely supported by H NMR spectral data and

crystallographic work, to the exclusive formation of the corresponding dpp complex [(dpp)Pd(OAc) ]·5H O (“py-pyz” coordination

site), this proving the tendency of dhdpp to generate dpp under diﬀerent reaction conditions. The promoted conversion of dhdpp

into dpp in the complex was examined by sequential NMR analysis and established to be determined by Pd(OAc) which plays the

role of catalyst. The new salt-like species [(CH )(dhdpp)PdI ](I)·7H O, prepared starting from [(dhdpp)PdCl ] in its reaction with

CH I, allowed the separation from the mother liquors of small brown crystals identiﬁed on the basis of X-ray analysis as the already

known complex of formula [(dpp)PdI ] (“py-py” coordination), this result once again outlining the tendency of dhdpp to be

dehydrogenated to dpp.

INTRODUCTION

It is well known that the N donor properties of pyridine in

terms of binding metal centers are stronger than those of

pyz” coordination), speciﬁc coordination depending, although

not exclusively, on the type and oxidation state of the metal

center.

■

Focusing on the series of known mononuclear complexes of

pyrazine due to its higher basicity (pyridine, pK = 5.5;

pyrazine, pK = 0.65). It is of fundamental interest to

dpp having formula [(dpp)MX

] with M = Pd or Pt recently

investigate the coordination sites binding to a metal center in

the complexes formed by multidentate N-donor ligands. The

concomitant presence of two vicinal pyridine rings and one

pyrazine ring in 2,3-di(2-pyridyl)-1,4-pyrazine (dpp, Chart 1A)

and in related species having either open chains or annulated

ring substituents in 5,6-positions has been considered as to the

formation of metal derivatives during the last 60 years, and this

reported and structurally elucidated by X-ray work (see Table

3a−f

1 in ref 2), the observed types of coordination, i.e., “py-py”

g−l

and “py-pyz”,

are also comparable. Our work in the area

was conﬁned to the synthesis and characterization of Pd and

Pt mononuclear derivatives of a substituted dpp, i.e., 2,3-

dicyano-5,6-di-(2-pyridyl)-1,4-pyrazine, [(CN) dpp] (Chart

work has been recently reviewed. As it appears evident from

Received: March 6, 2020

the complete list of data (see Table 1 in ref 2), achieved

information on mononuclear metal complexes indicates as

comparable the number of complexes in which metal

coordination takes place at the N atoms of the two pyridine

rings (Chart 1B, “py-py” coordination) or by using one

pyridine N atom and one pyrazine N atom (Chart 1C, “py-

XXXX American Chemical Society

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Article

Chart 1. Schematic Representation of the Structure of dpp (A), [(CN) dpp] (D), and dhdpp (E) and of the py-py (B) and py-

pyz (C) Modes of Coordination

D). Crystallographic work on the two isostructural complexes

undergo a dehydrogenation process thus forming dpp. The

driving force of this process is in relationship with the energy

relaxation eﬀect which implies for the pyrazine ring in the

process dhdpp → dpp the achievement of rearomatization.

The ﬁrst description of this process consisting of the releasing

of two hydrogen atoms taking place in hot mesitylene (1,3,5-

trimethylbenzene) in the presence of a palladium/charcoal

catalyst was given by the authors who ﬁrst reported the

II 4,5

[{(CN) dpp}MCl ] (M = Pd , Pt ) (Figure 1) deﬁnitely

2 2

establishes that chelation of the metal center in both species is

of the type “py-py ”.

synthesis of dhdpp. As it will be shown, the process dhdpp →

dpp has been accurately taken into account in the present work

while dealing with the synthesis of dhdpp and its related

mononuclear complexes formed by its reaction with Pd and

Pt reactants.

EXPERIMENTAL SECTION

■

Solvents and reagents were commercially obtained from Carlo Erba,

Fluka, or Sigma-Aldrich and used as received unless otherwise

speciﬁed. Dimethyl sulfoxide (DMSO) was freshly distilled over CaH₂

before use. 2,3-Bis(2-pyridyl)-1,4-pyrazine (dpp) and 2,2′-pyridil

Figure 1. ORTEP front (a) and side (b) views (50% probability

ellipsoids) of [{(CN) dpp}PtCl ].

In the eﬀort to render more probable and encounter the

asymmetric “py-pyz” type of chelation, attention has been

were purchased from Sigma-Aldrich. [(DMSO) PtCl ] and

[(C H CN) MCl ] (M = Pd , Pt ) were prepared as reported in

focused in the present work on the formation of Pt and Pd

complexes of 2,3-dihydro-5,6-di(2-pyridyl)-1,4-pyrazine

dhdpp, Chart 1E), in which the dihydropyrazine ring replaces

the pyrazine ring present in dpp. Asymmetric chelation, i.e., the

py-pyz” mode of coordination, was supposedly thought to be

the literature.

Synthetic Procedures. 2,3-Di-(2-pyridyl)-5,6-dihydropyrazine,

dhdpp, and its Bis-Hydrated Derivative, dhdpp·2H O. Following the

(

previously reported synthesis of dhdpp, a solution of ethylenedi-

amine (1.10 mL, 9.98 mmol) in ethanol (2.40 mL) was added to a

solution of 2,2′-pyridil (2.10 g, 9.90 mmol) in the same solvent (15.0

mL). The mixture was kept to reﬂux in air for 1 h; during this time,

the initial yellow color of the solution changed to brown red. After

having been cooled to room temperature, the solution was kept in the

refrigerator for 24 h, and the suspended orange crystalline material

was separated by centrifugation, washed twice with ethanol, and

brought to constant weight under vacuum (10 mmHg, 996.0 mg,

yield 42.2%). Calcd for dhdpp, C H N : C, 71.17; H, 5.12; N,

23.71%. Found: C, 70.80; H, 5.63; N, 23.32%. IR (cm ): 3034 (w),

“

more competitive for dhdpp by the implied enhancement of

the N donor properties of the dihydropyrazine ring, and the

presented results are also viewed in a comparison with those

obtained in parallel species by using ligand dpp. Worth noting,

the coordination properties of dhdpp are fully open to

exploration, since in relation to the highly extended

coordination chemistry of mono-, di-, and multinuclear dpp

−

metal derivatives, to our knowledge the only one so far

982 (vw), 2941 (w), 2877 (vw), 2824 (w), 1622 (vw), 1579 (vs),

551 (m), 1469 (m-s), 1457 (m-s), 1426 (m), 1327 (m), 1284 (w-

previously reported dhdpp metal derivative is the Pt complex

[

(dhdpp)PtCl ], a species further explored in the present

m), 1230 (w-m), 1153 (w), 1093 (m-s), 1048 (vw), 1028 (w), 983

vs), 891 (w), 854 (w), 814 (m), 786 (s), 753 (w), 741 (m-s), 696

contribution.

(

It is of relevance to notice here that dhdpp shows a

tendency, under a variety of experimental conditions, to

(w), 623 (m), 575 (w), 559 (m-s), 498 (vw), 477 (w), 405 (m), 389

(w), 372 (w), 344 (w), 283 (s). UV−vis spectral data (λ, nm; log ε)

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

under vacuum (50.0 mg, yield 52.7%). Calcd for [(dpp)PtCl ]·3H O,

2 2

C H Cl N O Pt: C, 30.34; H, 2.91; N, 10.11; Pt, 35.95%. Found: C,

14 16 2 4 3

−

30.57; H, 2.25; N, 9.70; Pt, 36.50%. IR (cm ): 3415 (w, broad),

3095 (vw), 3064 (vw), 3037 (m-w), 1639 (vw), 1594 (w), 1572 (w-

m), 1554 (m), 1509 (vw), 1465 (w-m), 1438 (vvw), 1429 (w), 1407

(s), 1394 (vs), 1313 (vvw), 1286 (vvw), 1259 (vvw), 1236 (m), 1157

(s), 1121 (vw), 1090 (w), 1067 (w), 1058 (w), 1036 (w), 1018 (vw),

986 (w), 965 (vvw), 884 (vvw), 861 (m), 835 (vvw), 807 (w), 786

(s), 754 (m-s), 741 (m-s), 665 (w), 651 (w), 620 (w), 585 (m), 554

(w), 482 (vw), 455 (w), 432 (vvw), 406 (vw), 345 (m-s; ν_P_t₋_C_l), 325

(w). UV−vis spectral data (λ, nm; log ε) in CH CN: 275 (4.66), 339

(3.82), 405 (3.47); in CHCl

: 278 (4.49), 337 (3.78), 424 (3.45). H

NMR (300 MHz, CDCl ): δ/ppm = 10.05 (d, J = 3.1 Hz, 1H, Ha);

hydrated dhdpp. Calcd for dhdpp·2H O, C H N O : C, 61.75; H,

9.96 (d, J = 5.3 Hz, 1H, H6); 8.83 (d, J = 3.1 Hz, 1H, Hb); 8.70 (d, J

= 5.1 Hz; 1H, H6′); 8.08 (td, J = 1.7 Hz, 7.70 Hz, 1H, H4′); 7.99 (d,

J = 7.9 Hz, 1H, H3′); 7.80 (td, J = 1.5 Hz, 8.0 Hz, 1H, H4); 7.58 (m,

2H, H5, H5′); 7.02 (d, J = 7.9, 1H, H3).

14 16

−

.92; N, 20.57%. Found: C, 62.17; H, 5.60; N, 20.29%. IR (cm ):

312 (s), 3044 (vw), 2926 (vw), 1648 (vs), 1580 (w), 1560 (w),

520 (vs), 1457 (m), 1427 (m), 1316 (w), 1285 (m), 1247 (m), 1219

(

w), 1150 (w), 1078 (vw), 1040 (vw), 1024 (vw), 990 (m), 962 (vw),

80 (w), 815 (w), 789 (vw), 742 (m), 673 (s), 617 (m), 485 (w), 447

[(dhdpp)PdCl ]·2H O. The compound [(C H CN) PdCl ] (71.4

2 2 6 5 2 2

mg, 0.186 mmol) was added to the solution obtained by dissolving

dhdpp (44.0 mg, 0.186 mmol) in CH OH (2.0 mL) with a change of

(

vw), 403 (w), 369 (vw), 353 (vw), 330 (w-m). UV−vis spectral data

λ, nm; log ε) in CH CN: 216 (4.11), 263 (3.87); in CHCl : 263

the initial orange color of the solution to mustard. Keeping the

mixture under stirring for a few hours at room temperature led to the

formation of a light-brown microcrystalline material which was

3.97). The same reaction conducted as above with the same

quantities of reagents and identical reaction procedure conﬁrmed the

formation of comparable amounts of dhdpp (1.39 g) and dhdpp·

separated by centrifugation, washed twice with CH

OH, and brought

to constant weight under vacuum (10 mmHg, 61.40 mg, yield

79.7%). Calcd for [(dhdpp)PdCl ]·2H Pd: C,

37.40; H, 3.59; N, 12.46; Pd, 23.67%. Found: C, 37.93; H, 3.11; N,

−2

H O (43.5 mg). In a further attempt, under the same experimental

conditions, pure dhdpp was obtained (935 mg, yield 40.0%). The

mother liquors kept in the refrigerator for over 2 weeks led to the

formation of a solid material which was separated by centrifugation,

washed with ethanol, and brought to constant weight under vacuum

O, C14H16Cl N O

2 2 4 2

−

12.60; Pd, 22.93%. IR (cm ): 3528 (w-m), 3410 (w-m), 3090 (vw),

3060 (w), 3038 (w-m), 2926 (vw), 2829 (vw), 1636 (vw), 1605 (w),

1585 (w), 1575 (m), 1540 (w-m), 1462 (m), 1425 (w-m), 1332 (m),

1282 (w-m), 1237 (m), 1155 (w-m), 1087 (w), 1063 (w), 1029 (w-

m), 1016 (s), 988 (w-m), 906 (w), 858 (w), 803 (w-m), 776 (vs),

745 (m), 697 (vw), 656 (vw), 617 (w-m), 569 (m), 506 (vw), 435

(w-m), 411 (vw), 359 (w), 347 (m)/331 (m) (νPd−Cl). UV−vis

−

(

10 mmHg, 262.8 mg). In the solid, partly composed of dhdpp·

H O, thin yellow needles were also found present (approximately

0% of the total amount of material), one of them isolated and

examined by single-crystal X-ray analysis which established the

needles to be dpp (Table S1 ; see the following Discussion section).

Synthesis of the Complexes [(dhdpp)MX ]·xH O and Their

spectral data (λ, nm; log ε) in CH CN: 214 (4.61), 263 (4.19), 293,

Analogs [(dpp)MX ]·xH O (M = Pt , Pd ; X = Cl, OAc).

sh (3.97), 331 (3.67), 365 (3.54); in CHCl : 262, sh (4.24), 339

[

(dhdpp)PtCl ]·5H O. This complex was previously prepared and

(3.64), 372 (3.55). H NMR spectral data (300 MHz, CDCl ): δ/

formulated as [(dhdpp)PtCl ]·H O·0.3CH OH. Following the

ppm = 9.49 (d, J = 7.1 Hz, 1H, H6); 8.48 (d, J = 4.8 Hz, 1H, H6′);

8.11 (d, J = 7.8 Hz, 1H, H3′); 7.97 (td, J = 1.6 Hz, 7.70 Hz, 1H, H4′);

7.79 (td, J = 1.8 Hz, 7.00 Hz, 1H, H4); 7.62 (td, J = 1.2 Hz, 7.00 Hz,

1H, H5); 7.48 (m, 1H, H5′); 6.84 (d, J = 8.0 Hz, 1H, H3); 4.27 (t, J

= 7.3 Hz, 7.40 Hz; 2H, Ha); 3.96 (t, J = 7.7, 6.90; 2H, Hb). It has

been veriﬁed that the same complex was obtained with a procedure in

which the reaction mixture was heated with stirring at 60 °C for 1 h

(yield: 78.5%).

reported procedure, the diﬀerently formulated complex, i.e.,

(dhdpp)PtCl ]·5H O, was obtained by us as follows: addition of

[

dhdpp (28.2 mg, 0.119 mmol) to a solution of [(DMSO) PtCl ]

(

50.4 mg, 0.119 mmol) in CH OH (10.0 mL) determines a rapid

color change of the mixture from yellow to red. After 30 minutes, the

formed brown crystalline material was separated by centrifugation,

washed twice with CH OH, and brought to constant weight under

−

vacuum (10 mmHg, 44.0 mg, yield 73.6%). Calcd for [(dhdpp)-

The same species in its hydrated form [(dhdpp)PdCl ]·4H O was

PtCl ]·5H O,C H Cl N O Pt: C, 28.39; H, 3.74; N, 9.46; Pt,

also prepared using dhdpp and PdCl as reactants in the following

14 22

−

2.93%. Found: C, 28.14; H, 3.00; N, 9.00; Pt, 32.48%. IR (cm ):

527 (sh), 3414 (w-m), 3068 (w), 3040 (w), 1594 (w), 1574 (vw),

554 (vw), 1526 (vw), 1493 (vw), 1465 (m), 1412 (m), 1337 (w),

282 (w), 1239 (w), 1155 (w), 1088 (vw), 1060 (vw), 1017 (m-s),

86 (w), 906 (w), 858 (vw), 798 (w), 774 (s), 744 (w-m), 697 (vw),

62 (vw), 638 (vw), 616 (w), 568 (w-m), 514 (vw), 478 (vw), 444

procedure: PdCl (25.3 mg, 0.135 mmol) was added to a solution of

dhdpp (20.3 mg, 0.085 mmol) in CH CN (10 mL), and the mixture

was kept under stirring at room temperature for 20 h. The light brown

colored solid formed was separated by centrifugation from the mother

liquors, washed twice with CH CN, and brought to constant weight

under vacuum (15.9 mg, yield 45.2%). Calcd for [(dhdpp)PdCl ]·

(

w), 347 (sh)//335 (m) (ν_P_t₋_C_l). UV−vis spectral data (λ, nm (log

4H O, C H Cl N O Pd: C, 34.62; H, 4.15; N, 11.54; Pd, 21.91%.

14 20

ε)) in CH CN: 222, sh (4.06), 270 (5.97), 344 (3.56), 445 (3.50); in

Found: C, 34.91; H, 3.32; N, 11.20; Pd, 21.30%. H NMR spectral

resonance peaks for this species in CDCl₃are found perfectly

coincident with those already given for the sample prepared using

[(C H CN) PdCl ] as reactant (see above).

CHCl : 276 (3.95), 352 (3.51), 465 (3.56). H NMR (300 MHz,

CDCl ): δ/ppm = 9.95 (d, J = 5.0 Hz, 1H, H6); 8.49 (d, J = 4.7 Hz,

1H, H6′); 8.10 (d, J = 7.8 Hz, 1H, H3′); 7.98 (td, J = 1.7 Hz, 7.79 Hz,

1H, H4′); 7.83 (td, J = 1.2 Hz, 7.92 Hz, 1H, H4); 7.69 (td, J = 1.3 Hz,

5.86 Hz, 1H, H5); 7.49 (m, 1H, H5′); 6.86 (d, J = 8.1 Hz, 1H, H3);

4.64 (t, J = 6.5 Hz, 7.45 Hz, 2H, Ha); 3.92 (t, J = 7.4 Hz, 7.21 Hz, 2H,

[(dpp)PdCl ]·3H O. This complex, previously prepared and

a,g

formulated as [(dpp)PdCl₂],

was obtained with a diﬀerent

procedure conducted here as follows: a brown suspension of PdCl₂

Hb).

(dpp)PtCl ]·3H O. This complex, previously prepared and

(44.6 mg, 0.252 mmol) and dpp (60.0 mg, 0.256 mmol) in CH OH

(8.0 mL) was kept under stirring at room temperature for 20 h. The

mustard colored solid formed was separated from the mother liquors

[

3h,i

formulated as [(dpp)PtCl₂], was obtained as follows: a suspension

of (DMSO) PtCl (72.1 mg, 0.171 mmol) in CH OH (7.5 mL) was

by ﬁltration, washed twice with CH OH, and brought to constant

−

added to dpp under stirring (40.0 mg, 0.171 mmol), and the mixture

was heated at 60 °C for 2 h, with the color changing from the initial

light yellow to red. After having been cooled to room temperature, the

orange solid material was separated by centrifugation from the mother

weight under vacuum (10 mmHg, 60.5 mg, yield 51.6%). Calcd for

[(dpp)PdCl ]·3H O, C H Cl N O Pd: C, 36.11; H, 3.46; N, 12.03;

14 16

Pd, 22.85%. Found: C, 35.72; H, 2.84; N, 11.70; Pd, 23.40%. IR

−

(cm ): 3490 (m-w, broad), 3073 (w-m), 3046 (w-m), 1590 (m),

1554 (vw), 1509 (vvw), 1460 (m-s), 1447 (vvw), 1407 (vs), 1317

liquors, washed twice with CH OH, and brought to constant weight

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

(

vw), 1282 (vw), 1241 (m), 1161 (m-s), 1129 (vvw), 1108 (vvw),

062 (m), 1031 (w-m), 1018 (w-m), 986 (vvw), 973 (vvw), 911

vvw), 897 (vvw), 858 (w), 830 (vw), 807 (w), 799 (vw), 777 (s),

54 (w), 651 (w), 620 (vvw), 598 (m-w), 585 (w), 549 (w), 473 (w),

42 (vw), 419 (vw), 350 (s; ν_P_d₋_C_l), 307 (w), 286 (w). UV−vis

(m), 1554 (vw), 1460 (s), 1425 (m-s), 1330 (s), 1286 (w), 1251 (w),

1233 (m), 1157 (m), 1085 (w), 1062 (w), 1031 (m-s), 1021 (m-s),

1004 (m), 866 (vw), 790 (w), 772 (vs), 746 (w), 728 (w), 651 (vw),

611 (vw), 562 (vw), 518 (w), 478 (w), 437 (vw), 419 (w), 345 (w-

m), 317 (w-m). UV−vis spectral data (λ, nm; log ε) in CH CN: 208

spectral data (λ, nm; log ε) in CH CN: 209 (4.37), 276 (3.89), 327

sh (4.60), 247 (4.44). From the mother liquors kept in the

refrigerator for a long time a small amount of small brown colored

(

3.67); in CHCl : 277 (4.15), 330 (3.77). H NMR (300 MHz,

crystals was isolated of the complex of formula [(dpp)PdI ], as was

unequivocally established by single-crystal X-ray work (Table S1 ; see

Discussion section below).

X-ray Crystallographic Measurements. Single crystal X-ray

intensity data for all compounds were collected at room temperature

on a Bruker SMART 1000 CCD diﬀractometer equipped with

graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Data

collection and reduction were carried out using the APEX2 and

SAINT packages.₁

Multiscan absorption correction using the

SADABS software was applied to the intensity data. The structures

11b

were solved by direct methods using SHELXT

and reﬁned with

full-matrix least-squares on F on all unique reﬂections using

SHELXL-2018/3.

All the non-hydrogen atoms were reﬁned

anisotropically. The hydrogen atoms were placed geometrically and

reﬁned using a riding atom approximation, with C−H = 0.93−0.96 Å,

and with U (H) = 1.2U (C) or U (H) = 1.5U (C) for methyl H

iso

atoms. In complex [(dpp)Pd(OAc) ] a rotating model was used for

the methyl groups, and ISOR restraints were applied. The very poor

(

4.51), 333, sh (3.72). As shown by the following NMR spectral data,

quality of the crystals of [(dpp)Pd(OAc) ] available for data

the complex has been identiﬁed as a dpp derivative, and its correct

formulation is [(dpp)Pd(OAc) ]·5H O, also well supported by the

collection may account for the very high R_i_n_tand poor R factors

obtained in the reﬁnement.

above found elemental analyses (see the Discussion section below).

Sequential H NMR Analysis of dhdpp Conversion into dpp.

H NMR (300 MHz, CDCl ): δ/ppm = 8.84 (d, J = 2.9 Hz, 1H, Ha);

A 5 mm NMR tube was charged with dhdpp (14 mg, 0.0592 mmol),

.69 (d, J = 4.1 Hz, 1H, H6); 8.52 (d, J = 2.9 Hz, 1H, Hb); 8.50 (d, J

Pd(OAc) (1.6 mg, 0.00713 mmol) (molar ratio 8.3:1), and THF-d

2 8

6.1 Hz, 1H, H6′); 8.05 (td, J = 1.7 Hz, 7.8 Hz, 1H, H4′); 7.94 (d, J

(0.60 mL), then ﬂushed with nitrogen, and sealed with an NMR

pressure cap. The mixture was shaken just before introduction into

the NMR probe, thermostated at 24 °C. The sequence of spectra was

obtained with 30 min time intervals or longer at advanced reaction

stages. The spectra were analyzed by integration of the singlet signal

at 8.62 ppm (dpp) with respect to the solvent signal at 1.74 ppm,

taken as internal reference. Relative concentration of dhdpp and dpp

can be obtained by integration of the signals at 8.16 ppm (d, J = 4.2

Hz) and at 8.62 ppm, respectively.

7.7 Hz, 1H, H3′); 7.75 (m, 1H, H4); 7.59 (m, 1H, H5); 7.51 (m,

H, H5′); 6.99 (d, J = 8.2 Hz, 1H, H3); 2.19, 2.18 (6H, 2CH ).

[

(dpp)Pd(OAc) ]·2H O. This is a new Pd -dpp derivative directly

2 2

prepared as follows: a solution of Pd(OAc) (48.1 mg, 0.214 mmol)

in THF (4.0 mL) was added to a solution of dpp (49.0 mg, 0.209

mmol) in the same solvent (2.0 mL), and the resulting purple-red

solution was kept at room temperature for 3 h. The solid formed was

separated from the mother liquors by centrifugation, repeatedly

washed with THF, and brought to constant weight under vacuum

Other Physical Measurements. IR spectra were recorded on a

−

−1

(

10 mmHg, 59.3 mg, yield 57.3%). Calcd for [(dpp)Pd(OAc) ]·

Varian FT-IR 660 in the range 4000−250 cm (KBr pellets or nujol

H O, C H N O Pd: C, 43.69; H, 4.07; N, 11.32; Pd, 21.51%.

mulls between CsI disks). UV−vis solution spectra were recorded

18 20

−

Found: C, 43.38; H, 3.91; N, 10.74; Pd, 20.87%. IR (cm ): 3425 (w-

m), 3099 (vvw), 3055 (vw), 2984 (vvw), 2926 (vvw), 1641 (vs),

with a Varian Cary 5E spectrometer using 1 cm quartz cuvettes. H

NMR spectra were recorded on a Bruker 300 AvanceII spectrometer

operating at 300 MHz and referenced to the residual solvent signal

597 (s), 1564 (m), 1500 (vvw), 1474 (w-m), 1406 (s), 1360 (s),

312 (vs), 1298 (s), 1273 (m), 1161 (w), 1150 (w), 1074 (vw), 1061

(CHCl , δ = 7.26 ppm; CD CN, δ = 1.96 ppm). X-ray Diﬀraction

3 H 3 H

(

w), 1043 (w), 1007 (w), 993 (w), 912 (vw), 858 (vvw), 837 (vvw),

16 (vvw), 789 (m), 773 (w), 754 (w). UV−vis spectral data (λ, nm;

log ε) in CH CN: 270 (4.22), 325 (3.86), 527 (3.10); in CHCl : 266,

(XRD) patterns were obtained with a Philips PW 1729 diﬀractometer

using CuKα (Ni-ﬁltered) radiation in the 5−60° 2θ range (step size

0.02°; time per step 1.25 s). Elemental analyses for C, H, N, and S

were provided by the “Servizio di Microanalisi” at the Department of

sh (4.18), 325 (3.95), 432 (3.23), 536 (3.15); in H O: 269 (4.23),

26 (3.93). H NMR (300 MHz, CDCl ): δ/ppm = 8.83 (d, J = 2.8

Chemistry, Universita Sapienza (Rome), using an EA 1110 CHNS-O

Hz, 1H, Ha); 8.68 (d, J = 4.1 Hz, 1H, H6); 8.52 (d, J = 2.9 Hz, 1H,

Hb); 8.50 (d, J = 6.3 Hz, 1H, H6′); 8.04 (td, J = 1.8 Hz, 7.7 Hz, 1H,

H4′); 7.93 (d, J = 7.8 Hz, 1H, H3′); 7.73 (m, 1H, H4); 7.58 (m, 1H,

H5); 7.49 (m, 1H, H5′); 6.98 (d, J = 7.6 Hz, 1H, H3); 2.19, 2.18

instrument. The ICP-PLASMA Pd and Pt analyses were performed on

a Varian Vista MPX CCD simultaneous ICP-OES. GC-MS analyses

were obtained on an Agilent Technologies 6890N Network GC

System equipped with a 5973 Network Mass Selective Detector.

(

6H, 2CH ).

[

{(CH )dhdpp}PdI ](I)·7H O. The complex [(dhdpp)PdCl ]·

DISCUSSION

■

H O (20.0 mg, 0.044 mmol) was suspended in CH CN (5.0 mL),

and CH I was added (100.0 μL, molar ratio 1:33). The mixture was

As evidenced in the Introduction, the present project was

planned as an investigation on the modes of bidentate

chelation, either of the type “py-py” or “py-pyz”, present in

the mononuclear complexes formed by 5,6-di-(2-pyridyl)-2,3-

heated with stirring at 70 °C for 3 h, with the color of the suspension

changing from brown to dark red. After having been cooled to room

temperature, the mixture was kept in the refrigerator for 24 h. The

solid present in the suspension was separated from the mother liquors

dihydropyrazine (dhdpp) formulated as [(dhdpp)MX ] (M =

by centrifugation, washed twice with CH CN, and brought to

_c_o_n_s_t_a_n_t_w_e_i_g_h_t_u_n_d_e_r_v_a_c_u_u_m₍₁₀− mmHg, 12.0 mg, yield 44.8%).

Pd , Pt ; X = Cl, OAc), having as a base of information what

was previously reported on dpp and derivatives in a recent

Calcd. for the formula [{(CH )dhdpp}PdI ](I)·7H O,

review. During the development of this work, the tendency of

C H N I O Pd: C, 20.84; H, 3.38; N, 6.48; Pd, 12.31%. Found:

29 4 3

−

0.33; H, 3.12; N, 6.18; Pd, 12.90%. IR (cm ): 3600−3300 (broad,

w-m) 3064 (w-m), 3051 (w-m), 2997 (w), 2961 (w), 2867 (w), 1581

dhdpp to undergo the already mentioned process dhdpp →

dpp was accurately taken care of; as it will be shown, formation

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 2. ORTEP view (50% probability ellipsoids) of (a) dhdpp and (b) dpp.

of dpp has been found to take place under a variety of

experimental conditions.

stretching) and 1648 cm⁻(H O bending) suggesting a precise

location of H O in the crystalline phase of the species. The X-

Synthesis and Properties of 5,6-Di-(2-pyridyl)-2,3-

ray powder spectra of the two species (Figure S2 ) reveal a high

level of crystallinity. Comparison of the two spectra clearly

indicates that they are not isomorphous. Attempts to obtain

dihydropyrazine (dhdpp). The synthesis of dhdpp,

conducted as previously reported, led in our synthetic

procedure to two diﬀerent compounds, i.e., dhdpp and

crystals for dhdpp·2H O suitable for single crystal X-ray work

were not successful.

dhdpp·2H O. As established by previously reported X-ray

work, duplicated by us (data in Table S1, Supporting

Information) dhdpp shows (Figure 2a) a noncoplanar

arrangement of the two pyridine rings, both of them also

noncoplanar with the dihydropyrazine ring, this latter highly

far from the full planarity present instead in the aromatic

pyrazine ring of dpp (Figure 2b). The dissimilar structural

and electronic distribution present in the dihydropyrazine ring

and the parent pyrazine ring was thought to determine in the

ﬁrst one an enhancement of its own N donor properties, the

main object of research of the present work.

It is of great interest that in one synthetic procedure of

dhdpp (Experimental Section), thin yellow crystalline needles

were also formed in the mixture with a second component by

long-standing of the mother liquors. One of the crystals was

isolated and examined by single crystal X-ray work, which

established the needles to be the compound dpp (Figure 2 b;

crystal data in Table S1; H NMR spectrum in Figure S1B)

evidently formed from dhdpp as a result of the dehydrogen-

ation process dhdpp → dpp. The IR spectrum of the solid

crystalline mixture (Figure S3) indicates that the second

The IR spectra of dhdpp and dhdpp·2H O are shown in

component is dhdpp·2H

narrow peaks present at 3312 and 1648 cm due to H O (no

O, as clearly suggested by the two

−

−1

Figure 3 (region 3500−300 cm ). The spectrum of dhdpp

Figure 3A) is identical to that reported. The presence of

(

attempts to separate the two species were made). From these

data it has been possible to learn that formation of dpp from

dhdpp can take place in the absence of a catalyst, as was

instead the case for the above cited process determined by a

H O in the spectrum of dhdpp·2H O (Figure 3B) is well

−

evidenced by narrow intense peaks at 3312 cm (O−H

palladium/charcoal catalyst.

[

(dhdpp)MCl ]·xH O (M = Pt , Pd ). Among the present

2 2

dhdpp derivatives, the complex [(dhdpp)PtCl ]·5H O is

considered ﬁrst since it is the only one having a previously

reported analog useful for comparison and formulated as

[

(dhdpp)PtCl ]·H O·0.3CH OH, our compound appositely

similarly prepared although obtained with a diﬀerent

formulation, i.e., [(dhdpp)PtCl ]·5H O (Experimental Sec-

tion). The H NMR spectrum in CDCl of our complex

(

Figure 4) shows the presence of ten resonance peaks; eight of

them are present in the region of the aromatic protons (6−10

ppm): four of which (3, 4, 5, 6) are assigned to the H atoms of

the pyridine ring involved in the coordination to Pt , and the

other four (3′, 4′, 5′, 6′) are attributable to the protons of the

other pyridine ring. The two additional resonance peaks

present in the region 3.8−4.8 ppm are assigned to the protons

of the two CH groups present in the dihydropyrazine ring of

the complex. These spectral features perfectly coincide with

those previously reported, thus deﬁnitely conﬁrming for the

complex [(dhdpp)PtCl ] coordination of the type “py-pyz”.

These results clearly indicate that despite the diﬀerent

formulation, the presence in the two species of solvent

molecules does not aﬀect the intimate structural environment

in [(dhdpp)PtCl₂].

Figure 3. IR spectra of dhdpp (A) and dhdpp·2H O (B).

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 4. H NMR spectrum in CDCl of [(dhdpp)PtCl ]·5H O.

In a second test planned to examine the tendency of dhdpp

to coordinate in a “py-pyz” fashion, samples of the Pd analog

[

(dhdpp)PdCl ]·2H O were obtained using diﬀerent reaction

2 2

conditions in two distinct preparative procedures (Exper-

imental Section, yields ca. 80%). Elemental analyses in both

cases conﬁrm for the complex the given formulation. The X-ray

powder spectrum of the compound (Figure 5 A) shows a high

Figure 6. IR spectra of [(dhdpp)PdCl ]·2H O (A) and [(dhdpp)-

−

PtCl ]·5H O (B) in the range 3750−300 cm .

−

peaks at 3528 and 3410 cm due to H O, peaks due to C−H

−

bonds at ca. 3000 cm , and, worth being noticed, a composed

peak present at 347/331 cm assigned as ν_P_d₋_C_l, assignment

encouraged by the absorption positioned at 340/335 cm for

the structurally characterized complex [{(CN) (dpp)}PdCl ].

−

Similarly, the IR spectrum of the Pt species (Figure 6B)

Figure 5. X-ray powder spectra of [(dhdpp)PdCl ]·2H O (A) and

−

[

(dhdpp)PtCl ]·5H O (B).

shows a broad band in the region 3600−3300 cm with a

−1 −1

peak at 3414 cm and shoulder at 3527 cm , positions

practically coincident with those of the Pd analog, with

crystalline character. Comparison of the spectrum with that of

broadening in the region caused most probably by the three

additional disordered water molecules and peaks in the region

the Pt analog (Figure 5B) indicates isomorphism between the

−1

two species. This result strongly suggests that the Pd complex

at ca. 3000 cm also assigned to the stretching of the C−H

also involves the metal center in a “py-pyz” mode of

groups. Noteworthy, the observed narrow intense peaks at 347

−

coordination.

(sh) and 335 cm (Figure 6B, see inset) are assigned as ν_P_t₋_C_l,

The IR spectra of the Pd and Pt complexes are closely

similar in all regions explored (Figure 6). The spectrum of the

in line with a similar assignment made for the absorption

observed in an identical position for the complex of the known

Pd species (Figure 6A) shows two distinct absorptions with

structure having the formula [{(CN) dpp}PtCl ] (Figure 1).

Inorg. Chem. XXXX, XXX, XXX−XXX

(OAc) ]·5H O. The IR spectrum (Figure 8 ) is characterized

Figure 7. H NMR spectrum in CDCl of [(dhdpp)PdCl ]·4H O.

The established form of “py-pyz” coordination for the Pd^I^I

of coordination. These results inform that using our synthetic

procedure the observed “py-pyz” mode of coordination

duplicates for both species the one already known for the

and Pt complexes ﬁnds useful further support from the

comparison of their H NMR spectra. In fact, the H NMR

3a,d,h,i

spectrum of the Pd species (Figure S4 ) shows main peak

nonhydrated parent species.

resonance positions exactly as for the Pt analog. However,

Attempt of the Synthesis of a dhdpp Complex with

owing to the presence of numerous low intensity resonance

peaks due to small amounts of unknown species, the complex

Pd(OAc) . In our third attempt to verify the tendency of

dhdpp to give a “py-pyz” mode of coordination, the isolated

new species formed from the reaction of dhdpp with

^P^d⁽^O^A^c⁾2 is reproducibly obtained in THF under mild

reaction conditions (Experimental Section). The amorphous

character of the isolated material is clearly shown by its X-ray

powder spectrum (Figure S8 ). Elemental analyses are in

agreement for a compound having the formula [(dhdpp)Pd-

was also prepared under mild reaction conditions using PdCl

as the Pd reactant instead of [(C H CN) PdCl ]. The H

NMR spectrum of the new compound [(dhdpp)PdCl ]·4H O

(

Figure 7) indicates the complete absence of impurities again

conﬁrming the presence of a “py-pyz” mode of coordination. It

is also appropriate here to notice that the two diﬀerent used

reaction procedures (solvent, Pd reactant) lead to the same

2 2

form of coordination, thus supporting the tendency of dhdpp

to prefer the “py-pyz” coordination, similar to what was ﬁrmly

established for the Pt analog.

[

(dpp)MCl ]·3H O (M = Pt , Pd ). The two dpp analogs of

2 2

II II

the above Pd and Pt dhdpp complexes were obtained by us

upon reaction of dpp in methanol with PdCl₂or

(

DMSO) PtCl , respectively, as the hydrated species of

2 2

formula [(dpp)MCl ]·3H O (M = Pt , Pd ). The complexes

parallel the respective nonsolvated species [(dpp)MCl ]

previously reported and structurally characterized.

these latter species, the Pt complex [(dpp)PtCl ] is known

only in its “py-pyz” mode of coordination, whereas for the

Pd analog both forms, i.e., “py-py” and “py-pyz”, were

obtained “during the same preparative procedure”.

a,d,h,i

3h,i

3a,d

Both of

our new hydrated species, obtained with methods diﬀerent

from those previously reported, show X-ray powder spectra

Figure 8. IR spectrum of the expected [(dhdpp)Pd(OAc) ]·5H O.

2 2

(Figure S5 ) not indicative of an isostructural arrangement in

the solid state. Indeed, their IR spectra in the region 3750−300

by the presence of a broad intense absorption with maximum

−

−1

cm are closely similar (Figure S6), both showing an intense

at ca. 3500 cm due to the O−H stretching of H O and of

narrow peak assigned as ν

at 351 cm⁻for the Pd species

two intense peaks at 1586 (covering also the O−H bending)

M−Cl

−

−1

and at 345 cm for the Pt analog. Great help as to the type of

and 1393 cm assigned, respectively, to the stretching

metal-to-ligand coordination present in our two complexes

vibrations ν

and ν_C₋_O, close to those found for Pd(OAc)

CO

−1

[

(dpp)MCl ]·3H O is provided by their H NMR spectra. The

at 1603 cm (ν

) and 1427 cm (ν_C₋_O).

CO

general common features of the spectra (Figure S7)

The H NMR spectrum of the compound (Figure 9) shows

unequivocally indicate for both complexes the “py-pyz” mode

complete absence of the resonance peaks in the 3.8−4.2 ppm

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 9. H NMR spectrum in CDCl of the synthesized complex formulated as [(dpp)Pd(OAc) ]·5H O.

region expected for the H atoms of the two CH groups of the

Crystal data are listed in Table S1; selected bond lengths and

angles are given in Table S2. These results indicate that during

the synthetic procedure of the planned dhdpp derivative the

ligand has been integrally changed to dpp. These ﬁndings are

diﬀerent from those exposed above in the case of dpp formed

from dhdpp only in low yield.

dhdpp ligand. In addition, the region of the aromatic hydrogen

atoms (6.7−9.0 ppm), plausibly interpretable as due to the

presence of the “py-pyz” mode of coordination, is also

accompanied by the presence of two resonance peaks due to

the hydrogen atoms of a pyrazine ring (CHa, CHb in Figure

In the attempt to gain further insight into the transformation

dhddp → dpp, and in particular to understand if the process is

The inset shows two resonance peaks at 2.19 and 2.18 ppm

assigned to the CH groups present in the two OAc fragments.

catalytic in metal complex, the reaction of dhdpp with

The observed NMR spectral features clearly suggest the

hypothesis that a dhdpp → dpp process might have taken

place. For further support of this, the dpp analog was

appositely synthesized and obtained in the form of a bis-

hydrated species of formula [(dpp)Pd(OAc) ]·2H O (see the

Experimental Section ). The H NMR spectrum of a sample of

this dpp species (Figure S9 ) coincides perfectly with that of the

supposed pentahydrated dhdpp derivative (Figure 9), for

Pd(OAc)

in THF-d

has been followed in situ by H NMR

spectroscopy, at 24 °C. The spectrum obtained immediately

after mixing reveals the presence of a singlet signal at 8.62 ppm,

which, along with the other signals, is unequivocally

attributable to the C−H hydrogens of the pyrazine ring. In

the course of the reaction, additional signals uncertainly

attributable to [(dpp)Pd(OAc) ] become well distinguishable

in the aromatic region. The spectrum taken after 5 minutes and

the last one taken when the intensity of the singlet signal of

dpp did not increase further (4 days) are shown in Figure

11A,B. A sequence of four spectra including two of them taken

at intermediate times is shown in Figure S10. At the last

registration (Figure 11B), the GC-MS analysis of the organic

components in the reaction mixture indicated 3% and 97 mol

which the exact formulation is therefore [(dpp)Pd(OAc) ]·

H O (irrelevant in the two species the diﬀerent presence of

water). X-ray analysis of single crystals of the complex isolated

from the mother liquors in its nonsolvated form illustrates the

structural arrangement of the entire molecule (Figure 10).

of dhdpp and dpp, respectively, cleanly indicating (Figure

S11 ) that the dehydrogenation process proceeds with Pd-

(

OAc) behaving as a catalyst.

A plot of the percent conversion of dhdpp into dpp vs time

Figure 12) clearly identiﬁes one rapid exponential phase

inset) followed by a slower zero-order phase.

(

The initial rapid conversion dhdpp → dpp can be attributed

to the presence of Pd(OAc)₂in catalytic amounts, thus

resulting in the reaction of dhdpp proceeding under pseudo-

ﬁrst-order conditions. As the reaction develops further and

Figure 10. Molecular structure of [(dpp)Pd(OAc) ] with displace-

ment ellipsoids drawn at the 50% probability level.

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 11. H NMR (300 MHz, THF-d , 24 °C) spectral evolution of dhdpp in the presence of Pd(OAc) (molar ratio 8.3:1): A) initial spectrum

(

5 min) and B) ﬁnal spectrum (93 h).

Pd(OAc) is sequestered in the complex with dpp, as observed

coordination palladium species, hence prone to interaction

with dhdpp, should then arise from reversible ligand

dissociation processes. Among these is release of one ligand

from Pd by solvent exchange or a switch of the coordination

mode of dpp from bidentate to monodentate, which is possible

in solution as well as by separation of precipitate, the rate only

depends on a slower rate-determining step not involving

dhdpp, presumably the generation of an active palladium

acetate species. A complex mixture of molecular complexes can

be envisaged in solution during the course of the reaction,

among which are dimeric palladium acetate, 1:1 adducts

due to the intrinsic hemilabile character of the N,N′ ligand.

Such a Pd−N bond-breaking step, if slower than the overall

dehydrogenation reaction, would also be rate determining and

thus account for the dominant zero-order phase shown in the

Pd(OAc) (L) of either dhdpp or dpp, and also, due to the

molar excess of the nitrogen ligands, 1:2 adducts Pd-

(

OAc) (L) with L as monodentate N-donors. Low-

graph of Figure 12. Although the details of the mechanism by

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

acetate counteranion in the dhdpp → dpp transformation.

Palladium acetate or palladium complexes in the presence of

carboxylate salts are commonly used as dehydrogenation

catalysts. Although harsh conditions and stoichiometric

amounts of terminal oxidants are required, recent develop-

ments include milder conditions in a simple aerobic environ-

ment or even without oxidants. What is rather peculiar in the

experiment described here and worth pointing out is that

dehydrogenation of dhdpp initiated by Pd(OAc)₂does

proceed with high conversion and selectivity even in the

presence of bidentate and strongly coordinating donor sites in

the substrate, conditions believed to be incompatible with Pd

catalyzed oxidation methods.

20b

[

{(CH )dhdpp}PdI ](I)·7H O. [(dhdpp)PdCl ], proved

3 2 2 2

above to imply “py-pyz” coordination, was made to react

with CH I under drastic reaction conditions intentionally

promoting quaternization of the N atom of the pyridine ring

not involved in the coordination to Pd and the predictable

concomitant change PdCl → PdI . Formation of the isolated

fully iodinated species formulated on the basis of elemental

analysis as a salt-like species of formula [{(CH )dhdpp}PdI ]-

(

I)·7H O was expected, as the pyridine ring as such, not

involved in the coordination to PdCl , could easily undergo

Figure 12. Percent formation of dpp vs time for the reaction of dhdpp

−

quaternization of the N atom. This type of process is

substantiated by the fact that in the case of the somehow

related unmetalated 2,3-dicyano-5,6-di(2-pyridyl)-1,4-pyrazine,

(

14 mg, 0.099 mol L ) with Pd(OAc) (1.6 mg, 12 mol %) in THF-

d (0.60 mL), as followed by H NMR spectroscopy (24 °C).

[

(CN) Py Pyz], carrying two vicinal pyridine rings, contact

which palladium acetate catalyzes the dehydrogenation of the

dhdpp ligand should require further investigation, at present a

catalytic cycle of the reaction can be envisaged as outlined in

Scheme 1, based on typical organometallic palladium

with CH

I in DMF under very mild reaction conditions (4

days at room temperature) leads to the formation of the

related salt-like species [(CN) Py(2-Mepy)Pyz](I) (only one

pyridine atom quaternized), of which the structure was

elucidated by X-ray work.

The stable-to-air hydrated salt-like species, formulated as

[{(CH )dhdpp}PdI ](I)·7H O in good agreement with

elemental analyses, although explored by IR and UV−visible

spectra, needs further studies for its characterization in terms

of physicochemical properties and structural features. Deserv-

ing mention here is that during the examined reaction of

[(dhdpp)PdCl ] with CH I a small amount of brown colored

chemistry: i) hydrogen abstraction from a C(sp )−H bond

by the acetate ligand with C−Pd bond formation (step B) and

ii) β-hydrogen elimination to form the double bond (steps C).

With regard to step B, there are several examples of

carboxylate-assisted palladium-catalyzed direct C(sp )−H

functionalization, also for C−H bonds in α to a nitrogen

atom. The diﬀerent outcome of the stoichiometric reaction

between dhdpp and PdCl , giving rise to [(dhdpp)PdCl ]·

H O, discloses unambiguously the key role played by the

Scheme 1. Suggested Catalytic Cycle for Dehydrogenation of dhdpp by Pd(OAc)₂

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

crystals was isolated from the mother liquors kept in the

refrigerator for a long time (see the Experimental Section).

Crystallographic work identi ﬁ ed the species as [(dpp)PdI₂]

interesting further aspect of this work, i.e., the repeatedly

veriﬁed tendency of dhdpp, either in the absence or in the

presence of a catalyst, to release hydrogen and form dpp under

a variety of experimental conditions. As a further interesting

related result, it has been proved that also in the synthetic work

leading to the salt-like species [{(CH )dhdpp}PdI ](I), a

(Figure 13 , crystal data in Table S1 ), a compound previously

collateral process takes place in which once more dpp is

formed from dhdpp giving rise to the complex [(dpp)PdI₂],

the structure of which has been elucidated by single-crystal X-

ray work. These results clearly indicate that careful analysis is

required when dealing with experimental work involving

dhdpp. Further extensive work is needed to enlarge the

knowledge of the coordination chemistry of dhdpp in mono-,

di-, and multinuclear complexes.

ASSOCIATED CONTENT

sı Supporting Information

■

Figure 13. ORTEP view (50% probability ellipsoids) of the complex

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00699.

[

⁽^d^p^p⁾^P^d^I2^]^.

H NMR spectra of dhdpp, dpp, [(dhdpp)PdCl ]·5H O,

reported and showing the “py-py” mode of coordination. A

[(dpp)PdCl ]·3H O, [(dpp)PtCl ]·3H O, and [(dpp)-

2 2 2 2

tentative explanation of the formation of this species from the

Pd(OAc) ]·2H O; X-ray powder spectra of dhdpp,

2 2

dhdpp complex [(dhdpp)PdCl ] leads to suggest that dpp,

dhdpp·2H O, [(dpp)PdCl ]·3H O, [(dpp)PtCl ]·

2 2 2 2

once formed during the reaction, induces in the temporarily

3H O, and [(dhdpp)Pd(OAc) ]·5H O; IR spectra of

2 2 2

formed new species [(dpp)PdCl ] an immediate change of the

IR spectra of dpp in mixture with dhdpp·2H O,

type of coordination from “py-pyz” to “py-py” with

[(dpp)PdCl ]·3H O, and [(dpp)PtCl ]·3H O; GC-MS

concomitant conversion of PdCl₂to PdI , this evidently

analysis of the reaction between dhdpp and Pd(OAc)₂

(corresponding to ﬁnal spectrum of Figure 11B); and

tables reporting crystallographic and structural reﬁne-

preventing quaternization of even only one pyridine N atom by

CH I. The overall process leading to formation of [(dpp)PdI ]

is a third established example in the present work of a dhdpp

dpp process, with all three cases examined occurring in

diﬀerent chemical events. Incidentally, the conversion of PdCl₂

PdI in [dppPdI ] supports that the same change has taken

place leading to the formation of the above fully iodinated salt-

like species.

ment data of dpp, dhdpp, [(dpp)Pd(OAc) ], and

→

[(dpp)PdI ] and selected bond lengths and angles of

[(dpp)Pd(OAc) ] and [(dpp)PdI ] (PDF)

→

Accession Codes

CCDC 1958377−1958380 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

CONCLUSIONS

In the present work the multiple N donor ligand 2,3-di-(2-

pyridyl)-5,6-dihydropyrazine (dhdpp) has been also isolated

■

and characterized as a bis-hydrated derivative, dhdpp·2H O.

Unexpectedly, crystallization of dhdpp occasionally allowed

isolation from the mother liquors of a low amount of crystals

proved by X-ray analysis to be the related dehydrogenated

species 2,3-di-(2-pyridyl)-1,4-pyrazine (dpp) formed in the

direct dehydrogenation process dhdpp → dpp. Central work

was directed to deeply explore and deﬁne the type of

coordination between the ligand dhdpp and the metal center

AUTHOR INFORMATION

■

Corresponding Authors

Maria Pia Donzello − Dipartimento di Chimica, Universita

Roma Sapienza, 00185 Roma, Italy; orcid.org/0000-0001-

084-0369; Email: mariapia.donzello@uniroma1.it

Mauro Bassetti − Dipartimento di Chimica, Universita

di Roma

in the complexes of formula [(dhdpp)MCl ] (Pt , Pd ). It has

Sapienza, 00185 Roma, Italy; CNR - Istituto per i Sistemi

Biologici, Sede di Roma - Meccanismi di Reazione, 00185

Roma, Italy; orcid.org/0000-0002-8163-3234;

Email: mauro.bassetti@uniroma1.it

been ﬁrmly established, by X-ray powder, IR and H NMR

spectral data, that a “py-pyz” mode of coordination is favored

with respect to the fairly equilibrated presence of “py-py” and

“py-pyz” in the dpp analogs previously published and also

Corrado Rizzoli − Dipartimento di Scienze Chimiche, della Vita

studied in the present work. In our further attempt directed to

support the tendency to coordinate in a “py-pyz” fashion,

e della Sostenibilita Ambientale, Universita di Parma, I-43124

̀ ̀

Parma, Italy; orcid.org/0000-0002-4841-6123;

Email: corrado.rizzoli@unipr.it

dhdpp has been studied in its reaction with Pd(OAc) . It has

been observed that, during the course of the reaction, dhdpp

undergoes a change to dpp, the overall process leading to

Claudio Ercolani − Dipartimento di Chimica, Universita di

Roma Sapienza, 00185 Roma, Italy; Email: claudio.ercolani@

uniroma1.it

formation of the complex [(dpp)Pd(OAc) ], as established by

H NMR spectral data and crystallographic work. An

additional detailed NMR investigation also deﬁnitely indicates

Authors

Edoardo Raciti − Dipartimento di Chimica, Universita

Sapienza, 00185 Roma, Italy

that Pd(OAc) plays the role of catalyst in the observed dhdpp

di Roma

→

dpp conversion. This remarkable result introduces the

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(6) Granifo, J.; Vargas, M. E.; Rocha, H.; Garland, M. T.; Baggio, R.

Article

Elisa Viola − Dipartimento di Chimica, Universita

di Roma

Structural and coordinating properties of platinum(II) chloride

complexes with polypyridyl ligands. Single crystal X-ray structure of

Sapienza, 00185 Roma, Italy

Ida Pettiti − Dipartimento di Chimica, Universita

Sapienza, 00185 Roma, Italy

di Roma

[

Pt(ddpq)Cl ]·H O (ddpq = 6,7-dimethyl-2,3-di(2-pyridyl)-

2 2

quinoxaline). Inorg. Chim. Acta 2001 , 321 , 209 −214.

Noemi Bellucci − Dipartimento di Chimica, Universita di Roma

(7) Goodwin, H. A.; Lions, F. Tridentate chelate compounds. II. J.

Sapienza, 00185 Roma, Italy

Am. Chem. Soc. 1959, 81, 6415−6422.

Complete contact information is available at:

(8) Price, J. H.; Williamson, R. F.; Schramm, R. F.; Wayland, B. B.

Palladium(II) and platinum(II) alkyl sulfoxide complexes. Examples

of sulfur-bonded, mixed sulfur- and oxygen-bonded, and totally

oxygen-bonded complexes. Inorg. Chem. 1972, 11, 1280 −1284.

https://pubs.acs.org/10.1021/acs.inorgchem.0c00699

Notes

Madison, Wisconsin, USA, 2008. (b) Sheldrick, G. M. SHELXT -

9) Kharasch, M. S.; Seyler, R. C.; Mayo, F. R. Coo

Compounds of Palladous Chloride. J. Am. Chem. Soc. 1938, 60, 882−

84.

rdination

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

(

10) Posel, M.; Stoeckli-Evans, H. IUCrData 2016, 1, No. x161467.

■

(11) (a) Bruker APEX2, SAINT and SADABS; Bruker AXS, Inc.:

Financial support by the University of Rome La Sapienza

Progetto di Ricerca - Anno 2017 − RM11715C819222B8)

and by CNR-DISBA project NutrAge (7022) is gratefully

acknowledged. M.P.D. is grateful to the Consorzio Inter-

universitario di Ricerca in Chimica dei Metalli nei Sistemi

Biologici (CIRCMSB) for scientiﬁc support.

Integrated space-group and crystal-structure determination. Acta

Crystallogr., Sect. A: Found. Adv. 2015 , A71 , 3−8. (c) Sheldrick, G.

M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect.

C: Struct. Chem. 2015, C71, 3−8.

(

12) Huang, N.-T.; Pennington, W. T.; Pedersen, J. D. Structure of

,3-bis(2-pyridyl)pyrazine. Acta Crystallogr., Sect. C: Cryst. Struct.

Commun. 1991, 47, 2011−2012.

13) Cook, A. K.; Sanford, M. S. Mechanism of the Palladium-

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