A study on the coordinative versatility of the zwitterionic S,N,S ligand EtNHC(S)Ph2P=NPPh2C(S)NEt in its anionic, neutral and cationic forms - determination of absolute pKa Values in CH 2Cl2 of Rh<su

Article abstract of DOI:10.1002/ejic.200800075

The coordination properties of EtNHC(S)Ph₂P=NPPh ₂C(S)-NEt (HEtSNS) towards Rh_I species derived from [Rh(CO)₂-Cl]₂ and [Rh(cod)Cl]₂ (cod = 1,5-cyclooctadiene) were studied. This ligand is a

Full text of DOI:10.1002/ejic.200800075

FULL PAPER
DOI: 10.1002/ejic.200800075
A Study on the Coordinative Versatility of the Zwitterionic S,N,S Ligand
EtNHC(S)Ph2P=NPPh2C(S)NEt in Its Anionic, Neutral and Cationic
Forms – Determination of Absolute pKa Values in CH2Cl2 of RhI Complexes
Massimiliano Delferro,[a] Daniele Cauzzi,*[a] Roberto Pattacini,[a] Matteo Tegoni,[a]
Claudia Graiff,[a] and Antonio Tiripicchio[a]
Keywords: Rhodium / Zwitterions / Coordination Modes / Acid–base properties
The coordination properties of EtNHC(S)Ph2P=NPPh2C(S)-
NEt (HEtSNS) towards RhI species derived from [Rh(CO)2-
diffraction methods. The [Rh(CO)EtSNS] complex is a bipro-
tic base; its conjugated acids are [Rh(CO)HEtSNS]+ and
[Rh(CO)H2EtSNS]2+, and their pKa values were determined
in dichloromethane solutions. In this triad of compounds,
which are geometrically similar, the RhI metal center features
variable charge density as confirmed by the νCO infrared
absorption frequency.
Cl]2 and [Rh(cod)Cl]2 (cod
= 1,5-cyclooctadiene) were
studied. This ligand is an amphoteric zwitterion, which forms
the H2EtSNS+ cation upon protonation and the EtSNS– di-
anion–cation upon deprotonation. All three forms coordinate
to metal centers. Their geometrical versatility allows many
coordination fashions: S-monodentate; S,S-bidentate (with a
bite angle spanning from 90 to 180°); S,N,S-tridentate;
N,N,N-tridentate; and S,S-bridging, as determined by X-ray
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
The zwitterionic ligand EtNHC(S)Ph2P=NPPh2NEt
(HEtSNS, Scheme 1) can be prepared in high yield by
double addition of ethylisothiocyanate (EtNCS) to bis(di-
phenylphosphanyl)amine (Ph2PNHPPh2, dppa),[1,2] which
forms one thioamidic and one thioamidyl functionality
bound to the phosphorus atoms. The opposite charges are
formally located on the zwitterionic thioamidyl–phospho-
nium, N–C(S)P+, functional group. In solution, a rapid pro-
ton exchange between the thioamidic–thioamidyl function-
alities is present. This exchange is facilitated by the forma-
tion of a pseudoring as a result of an intramolecular
N···H···N hydrogen bond, as found in the solid-state struc-
ture.[1] Thus, the positive charge can be shared by the two
P atoms of the P–N–P group and the negative charge can
be delocalized on the two EtNC(S) groups (Scheme 1).
As previously reported, EtSNS– acts as an S,N,S-triden-
tate or S,S-bidentate ligand towards MI species (M = Rh[1]
and Cu, Ag, Au[2]). The [–Ph2PNPPh2–]+ nitrogen atom can
be coordinated as in [Rh(CO)(EtSNS)] and [Cu(EtSNS)]
and it weakly interacts with the metal center in [Ag-
(EtSNS)]2, or it can be not coordinated as in the case of
[Au(EtSNS)]. The [–Ph2PNPPh2–]+ group is reminiscent of
Scheme 1. Lewis structures and formal charge distribution of
HEtSNS and its deprotonated (EtSNS–) and protonated
(H2EtSNS+) forms.
the classical bis(triphenylphosphane)iminium cation
[Ph3PNPPh3]+ (PPN+), whose N atom, as evidenced by its
bent geometry, may coordinate a metal center, although this
behavior has never been observed.[3] We also became inter-
ested in studying the coordination properties of HEtSNS
and H2EtSNS+. Their reactivity towards RhI species de-
rived from [Rh(CO)2Cl]2 and [Rh(cod)Cl]2 (cod = 1,5 cyclo-
octadiene) is reported. Acid–base properties of the [Rh-
(CO)(EtSNS)] complex are discussed together with a gene-
ral method for the determination of absolute pKa values in
dichloromethane.
[a] Dipartimento di Chimica Generale ed Inorganica, Chimica
Analitica, Chimica Fisica, Università degli Studi di Parma,
Viale G.P. Usberti 17/A, 43100, Parma, Italy
Fax: +39-0521905557
E-mail: cauzzi@unipr.it
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
2302
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2302–2312

Determination of Absolute pKa Values of RhI Complexes
Results and Discussion
Acid–Base Properties of HEtSNS
The thioamidyl moiety HEtSNS can be protonated with
acids such as HCl or HPF6 to form the respective salts,
whereas deprotonation of HEtSNS can be achieved with
NaH (Scheme 1).
Single crystals of (H2EtSNS)PF6 (yellow) and [Na-
(EtSNS)H2O]2 (colorless) were obtained. Views of their
crystal structures are reported in Figures 1 and 2, respec-
tively. Selected bond lengths and angles are listed in Table 1
and compared to those previously reported for HEtSNS[1]
and to those of the complexes described herein.
Figure 2. ORTEP plot of the structure of [Na(EtSNS)H2O]2. Ther-
mal ellipsoids are drawn at the 30% probability level. H atoms are
omitted for clarity.
ation from the mean plane defined by C1, N1, S1, C2, N2,
S2, P1, N3, P2: 0.104 Å], probably due to a bifurcated (on
the donor atom) hydrogen bonding of the type N1–
H···N3···H–N2 [H1···N3 and H2···N3 distances: 2.396(1)
and 2.418(1) Å]. The presence of an interaction with the N3
lone pair of electrons is also suggested by the P–N–P angle,
which is significantly narrower than those of HEtSNS and
[Na(EtSNS)H2O]2.
The hydrated dimeric salt [Na(EtSNS)H2O]2 crystallized
in air by evaporation of a CH2Cl2/hexane solution of [Na-
(EtSNS)]. Each Na+ cation interacts with the three nitrogen
atoms of EtSNS– [Na–N3, Na–N1, Na–N2 distances:
2.592(2), 2.478(2), 2.503(3) Å, respectively] and with the
oxygen atom of a water molecule [Na–Ow distance:
2.277(3) Å]. The two units are connected by S···Na interac-
tions [Na–S1Ј distance is 2.893(1) Å], which results in a dis-
torted square pyramidal geometry around Na+. It is worthy
to note that sulfur-bridged complexes of alkaline metals are
far less common than their oxygen-containing counter-
Figure 1. ORTEP plot of the molecular structure of (H2EtSNS)-
PF6. Thermal ellipsoids are drawn at the 30% probability level. H
atoms are omitted for clarity. Only one of the two crystallographic
independent (H2EtSNS)PF6, units is depicted.
In the crystals of (H2EtSNS)PF6, two independent parts.[4]
–
H2EtSNS+/PF6 couples are present and display similar
Solid Na(EtSNS) is not stable towards hydrolysis. When
geometrical parameters. The N1···N2 separation of left in the air, it reacts with water vapor and transforms into
4.315(3) Å is larger than that of HEtSNS [2.856(8) Å];[1] the HEtSNS. The presence of coordinated water in the crystals
latter features an intramolecular hydrogen bond of [Na(EtSNS)H2O]2 seems to indicate that the dimeriza-
(Scheme 1). Differently from HEtSNS, the P–N–P system tion results in a much lower reactivity towards hydrolysis.
is coplanar with the two thioamidic groups [maximum devi- When a sample of Na(EtSNS) was dissolved in CDCl3,
Table 1. Selected bond lengths [Å] and angles [°].
[Na(EtSNS)H2O]2
HEtSNS
(H2EtSNS)PF6
1
4
6
[Rh(CO)(EtSNS)]
8
7
P1–N3
P2–N3
P1–C1
P2–C2
C1–S1
1.576(2)
1.585(2)
1.845(2)
1.843(2)
1.713(2)
1.705(3)
1.276(3)
1.284(3)
–
1.584(2)
1.575(2)
1.841(2)
1.858(2)
1.690(2)
1.660(2)
1.297(3)
1.314(3)
–
1.596(1)
1.599(1)
1.843(2)
1.837(2)
1.650(2)
1.638(2)
1.302(3)
1.291(3)
–
1.5810(18)
1.5798(18)
1.831(3)
1.840(2)
1.747(3)
1.744(2)
1.269(3)
1.269(3)
2.3457(7)
2.4047(6)
–
1.5832(18)
1.5721(17)
1.829(2)
1.852(2)
1.747(2)
1.656(2)
1.266(3)
1.303(3)
2.3614(15)
–
1.580(2)
1.591(3)
1.843(3)
1.833(3)
1.631(3)
1.639(3)
1.305(4)
1.314(4)
–
1.643(2)
1.643(2)
1.851(5)
1.851(5)
1.764(5)
1.764(5)
1.288(7)
1.288(7)
2.367(2)
2.367(2)
2.159(2)
2.158(5)
1.625(2)
1.613(2)
1.823(3)
1.827(3)
1.742(3)
1.679(3)
1.279(3)
1.315(4)
2.315(1)
2.314(1)
2.126(2)
2.126(2)
1.619(1)
1.619(1)
1.825(2)
1.825(2)
1.673(2)
1.673(2)
1.307(3)
1.307(3)
2.298(1)
2.298(1)
2.115(3)
2.115(3)
C2–S2
C1–N1
C2–N2
S1–Rh1
S2–Rh1
N–Rh
–
–
–
–
–
–
–
–
–
P1–N3–P2
140.8(1)
140.3(3)
132.9(1)
136.21(12)
140.25(12)
136.71(16)
Eur. J. Inorg. Chem. 2008, 2302–2312
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2303

M. Delferro, D. Cauzzi, R. Pattacini, M. Tegoni, C. Graiff, A. Tiripicchio
FULL PAPER
spontaneous retransformation in H(D)EtSNS was moni-
tored by 31P{1H} NMR spectroscopy over a period of ca.
2 h; the 8.0 ppm signal of colorless Na(EtSNS) was re-
placed by that of yellow HEtSNS at 8.4 ppm.[5] The initial
observation of a sharp singlet (regardless of the tempera-
ture) suggested that, in this solution, the salt is present in its
monomeric form. On the contrary, monomeric NaEtSNS is
stable in DMSO solution. Also in this case, only a sharp
singlet was observed in the 31P{1H} NMR spectrum in
[D6]DMSO (δ = 7.2 ppm) and even the addition of a large
excess of water did not result in the formation of HEtSNS
(δ = 13.8 ppm). This stabilizing effect probably stems from
the interaction of Na+ with DMSO through the oxygen
atom.[6]
In (H2EtSNS)PF6, [Na(EtSNS)H2O]2, and HEtSNS, the
P1–N3 and P2–N3 bond lengths are rather similar, whereas
variations can be evidenced in the C–S and C–N distances.
As expected, the protonation of the thioamidyl group re-
sults in the lengthening of the corresponding C–N bonds
and in the shortening of the C–S distances; the same behav-
ior is observed in the complexes described herein.
Figure 3. ORTEP plot of the molecular structure of 1. Thermal
ellipsoids are drawn at the 30% probability level. H atoms are omit-
ted for clarity.
Coordination Properties and Geometrical Versatility of
EtSNS–, HEtSNS, and H2EtSNS+[7]
Reaction of HEtSNS with [Rh(CO)2Cl]2 in the Presence
of tBuOK
The reaction of HEtSNS and [Rh(CO)2Cl]2 in the pres-
ence of a stoichiometric amount of tBuOK results in the
quantitative formation of [Rh(CO)(EtSNS)]. This com-
pound was previously reported as the product of the reac-
tion between NaEtSNS and [Rh(CO)2Cl]2.[1b] In the same
work, [Rh(CO)2(S,N-EtSNS)] was proposed as an interme-
diate product of the reaction (evidenced by FTIR spectro-
photometry). In view of the isolation of compound 1, we
now propose [Rh(CO)2(S,S-EtSNS)] to be the actual inter-
mediate, as shown in Scheme 3.
Reaction of HEtSNS with [Rh(cod)Cl]2 in the Presence of
tBuOK
The reaction of HEtSNS and [Rh(cod)Cl]2 in the pres-
ence of a stoichiometric amount of tBuOK (Scheme 2) af-
fords [Rh(cod)(S,S–EtSNS)] (1) in quantitative yields. A
view of its molecular structure is depicted in Figure 3.
Scheme 2.
In complex 1, EtSNS– chelates the metal center through
both sulfur atoms to form an eight-membered ring. The
distorted square-planar coordination is completed by a che-
lating cod molecule. The S1–Rh1–S2 angle of 92.90(2)° is
dramatically narrower than that observed in the S,S-che-
lated complex [Au(EtSNS)],[2] in which the coordination
over the metal center is almost linear [S1–Au1–S2
177.73(6)°]. An intermediate geometry[2] was found in di-
mer [Ag(EtSNS)]2 [mean value of the S–Ag–S angles
136.6(1)°].[8] The structural flexibility of EtSNS– allows the
Scheme 3.
Complex 1 undergoes cyclooctadiene substitution by
bubbling CO in dichloromethane solution at room tempera-
ture and atmospheric pressure, which results in its quantita-
tive transformation into [Rh(CO)(EtSNS)].
A Rh2 Dinuclear Complex
Reaction of 1 with [Rh(cod)(thf)2]OTf results in the for-
ligand to comply with the coordination requests of the dif- mation of the purple dinuclear compound [{Rh(cod)}2(µ-
ferent metal centers. In 1, the charge separation is conserved S,S-EtSNS)]OTf (2). An ORTEP plot of the structure of
so that this complex belongs to the category of zwitterionic [{Rh(cod)}2(µ-S,S-EtSNS)]+ in 2·1/2thf is reported in Fig-
metalates.[9]
ure 4.
2304
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2302–2312

Determination of Absolute pKa Values of RhI Complexes
Reaction of HEtSNS with [Rh(cod)Cl]2
The [Rh(S-HEtSNS)(cod)Cl] (4) complex, in which
HEtSNS behaves as a monodentate ligand, was prepared
by treating HEtSNS with [Rh(cod)Cl]2 (Scheme 4).
Scheme 4.
In the 31P{1H} NMR spectrum of 4 (CDCl3), two sets
of peaks are present. Two doublets [δ = 11.1 (d), 10.5 (d)
ppm; 2JP,P = 15.9 Hz] can be assigned to 4, whereas a singlet
at δ = 12.4 ppm suggests the presence of a second species,
probably isomer [Rh(S,SЈ-HEtSNS)(cod)]Cl (4a), which
may be in slow equilibrium with 4 (integral ratio 4/4a ≈
10; see Figure S1, Supporting Information). In complex 4a,
HEtSNS could behave as an S,S-chelating ligand in a man-
ner similar to that observed in 1.
Figure 4. ORTEP plot of the molecular structure of the cation of
2 in 2·1/2thf. Thermal ellipsoids are drawn at the 30% probability
level. H atoms are omitted for clarity. Selected distances [Å] and
angles [°]: P1–N3 1.591(4), P2–N3 1.587(4), P1–C1 1.834(5), P2–
C21.829(5), C1–S1 1.786(5), C2–S2 1.790(5), C1–N1 1.244(6), C2–
N2 1.250(6), S1–Rh1 2.417(1), S2–Rh1 2.400(1), S1–Rh2 2.369(1),
S2–Rh2 2.407(2), Rh1–Rh2 2.942(1); P1–N3–P2 128.0(3).
Isomer 4a is only present in solution, as the HATR infra-
red spectrum of the crude powder obtained by evaporation
of the reaction solvent was found superimposable to the
1
spectrum of a single crystal of 4. In the H NMR spectra,
two broad N–H signals can be observed at δ = 11.4 and
12.8 ppm, (assigned to 4 and 4a, respectively). In the solid-
state structure of 4, HEtSNS is coordinated through the
sulfur atom of the thioamidyl group and a chloride atom,
and a chelating cod is bound to the Rh atom. An
N2H3n···Cl interaction is present, which forms a 10-mem-
bered pseudoring (Figure 5).
Dinuclear complex 2 can be seen as an adduct between
1 and fragment [Rh(cod)]+. Both sulfur atoms of EtSNS–
bridge the metal centers to form a Rh2S2 core, which is
rather common for dithiolate complexes.[10] The Rh–S bond
lengths range from 2.369(1) Å for Rh2–S1 to 2.417(1) Å for
S1–Rh1. The ligand geometry differs from that observed in
compound 1, which emphasizes its geometrical versatility.
In particular, the P1, C1, S1, N1 and P2, C2, S2, N2 mean
planes are almost orthogonal in 1 [88.2(1)°], whereas in 2
these planes are almost parallel [19.2(4)°].
A similar reaction between 1 and [Rh(cod)Cl]2 carried
out in chlorinated solvents did not afford the expected dinu-
clear complex [{Rh(cod)}2(µ-S,S-EtSNS)]Cl (3), analogous
to 2, as evidenced by the 31P{1H} NMR spectra, in which
only the signal of 1 (δ = 9.7 ppm, CDCl3) was visible.
In contrast, 3 was quantitatively formed when methanol
was used as solvent, as confirmed by 31P{1H} NMR spec-
troscopy and MS (ESI); the MS spectrum showed the ex-
pected [M – Cl]+ peak at m/z = 980.26. The assembly of 3
was monitored by 31P{1H} spectroscopy by the addition of
known quantities of CH3OH into a stoichiometric solution
of 1 and [Rh(cod)Cl]2 in CD2Cl2 (CD2Cl2/CH3OH ≈ 5).
Any attempt to obtain 3 in the solid state failed: reforma-
tion of 1 and [Rh(cod)Cl]2 was observed upon solvent re-
moval. Moreover, only crystals of the two precursors could
be separated by any crystallization method that was tried.
This equilibrium strongly depends on the dielectric proper-
ties of the solvent but also on the ionic strength of the solu-
tion, because upon addition of (Et4N)PF6, the assembly of
3 can be achieved in CHCl3.
Figure 5. ORTEP plot of the molecular structure of 4. Thermal
ellipsoids are drawn at the 30% probability level. H atoms are omit-
ted for clarity.
Eur. J. Inorg. Chem. 2008, 2302–2312
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2305

M. Delferro, D. Cauzzi, R. Pattacini, M. Tegoni, C. Graiff, A. Tiripicchio
FULL PAPER
As evidenced by in situ 31P{1H} NMR spectroscopy, 15.2 Hz) suggest the formation of the monodentate com-
when tBuOK was added to a CDCl3 solution of 4, com- plex [Rh(CO)2(S-HEtSNS)Cl], which is analogous to 4, as
pound 4a was first quickly and completely consumed, an intermediate product. Compound 5 was separated by
which was slowly followed by deprotonation of compound crystallization, and its molecular structure was determined
4. Both reactions converged to the formation of by X-ray diffraction. The cation in compound 5 stems from
[Rh(cod)(S,S-EtSNS)] (1, singlet at 9.7 ppm).
the protonation reaction of HEtSNS due to the scavenging
This fast reactivity is in agreement with the proposed of HCl liberated in the formation of [Rh(CO)-
structure of 4a, whose formula can be written as 1·HCl.
(EtSNS)]. The chloride ion was found coordinated by the
When methanol was added to a chloroform solution of Rh atom of [Rh(CO)2Cl2]–. The reaction is probably driven
4 (Scheme 5), the solution color instantly changed from yel- by the stability of the S,N,S-tridentate complex [Rh(CO)-
low to purple. The 31P{1H} NMR spectrum suggested the (EtSNS)]. Complex [Rh(CO)2(S-HEtSNS)Cl] can be re-
formation of H2EtSNS+ along with dinuclear compound 3 garded as the first reaction product, which successively
(13.3 and 17.9 ppm, respectively).
yields 5 and [Rh(CO)(EtSNS)]. An ORTEP view of the X-
ray molecular structure of 5 is reported in Figure S3 (Sup-
porting Information), and it displays geometrical param-
eters analogous to those observed for compound 6 (vide
infra).
Reaction of (H2EtSNS)X (X = Cl, PF6) with [Rh(cod)Cl]2
The reactivity of (H2EtSNS)X (X = Cl, PF6) depends on
the coordinative properties of the anion. The reaction of
(H2EtSNS)Cl with [Rh(cod)Cl]2 yielded [H2EtSNS]-
[Rh(cod)Cl2] (6). A view of its molecular structure, deter-
mined by X-ray diffraction methods, is reported in Figure 6.
Scheme 5. Solvent dependent equilibrium for complex 3 and 4.
This equilibrium, which implies the autoprotonation re-
action: 2 HEtSNSǞH2EtSNS+ + EtSNS– is somehow sim-
ilar to the formation of 3 in methanol, which confirms that
the cationic dinuclear entity [{Rh(cod)}2(µ-S,S-EtSNS)]+ is
favored in polar solvents. Moreover, attempts to perform
chloride abstraction from 4 by using AgOTf with the aim
of obtaining a S,SЈ-HEtSNS chelated complex analogous
to 4a failed; this reaction only resulted in the formation of
[{Rh(cod)}2(µ-S,S-EtSNS)]OTf (2).
Reaction of HEtSNS with [Rh(CO)2Cl]2
In the reaction of HEtSNS with [Rh(CO)2Cl]2 in di-
chloromethane (Scheme 6), three products are formed, as
detected by 31P{1H} NMR spectroscopy (see Figure S2,
Supporting Information).
Figure 6. ORTEP plot of the molecular structure of 6. Thermal
ellipsoids are drawn at the 30% probability level. H atoms are omit-
ted for clarity.
In the crystal structure, bond lengths and angles of
H2EtSNS+ are similar to those observed for (H2EtSNS)-
PF6, except for the P–N–P angle, which is slightly bigger [6:
136.7(2)°; (H2EtSNS)PF6: 132.9(1), 133.9(1)°], and the
value of the C1–P1–P2–C2 torsion angle [ca. 1° for
(H2EtSNS)PF6 and 91° for 6]. The cation–anion interaction
features two NH···Cl hydrogen bonds as depicted in Fig-
ure 6. The positive charge is located on the P–N–P system,
and this interaction is reminiscent of the anion coordina-
tion by neutral thioureas and ureas.[11] No reaction was ob-
Scheme 6.
The main products of the reaction correspond to the served between (H2EtSNS)PF6 and [Rh(cod)Cl]2. The
S,N,S-tridentate complex [Rh(CO)(EtSNS)] (δ = 16.6 ppm) H2EtSNS+ cation is thus unable to open the chloride bridge
and [H2EtSNS]+[Rh(CO)2Cl2]– (5; δ = 13.3 ppm). Two low- of the dimeric precursor and coordinate to the Rh center
intensity doublets at δ = 11.9 and 11.4 ppm (2JP,P
=
(see below for the coordination properties of H2EtSNS+).
2306
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2302–2312

Determination of Absolute pKa Values of RhI Complexes
Reaction of (H2EtSNS)X (X = Cl, PF6) with [Rh(CO)2Cl]2 or by treating [Rh(CO)2(MeCN)2]PF6 with (H2EtSNS)PF6.
The latter reaction is an example of complex formation by
Although 6 can be quantitatively prepared by the afore-
treating a cationic ligand with a cationic metal fragment,
mentioned reaction of (H2EtSNS)Cl with 1/2[Rh(cod)Cl]2,
which is not frequent in coordination chemistry.[12] The cat-
a similar reaction pathway cannot be followed for the syn-
ions of 7 and 9, [Rh(CO)(S,N,S-H2EtSNS)]+, are identical.
thesis of 5.
Single crystals of 8·2thf were obtained. Its structure is de-
picted in Figure 8.
The reaction of (H2EtSNS)Cl with [Rh(CO)2Cl]2 in
CH2Cl2 afforded [Rh(CO)(S,N,S-H2EtSNS)][Rh(CO)2Cl2]2
(7); two unreacted (H2EtSNS)Cl molecules were also ob-
served by 31P{1H} NMR spectroscopy. The coordination of
the Cl– ion liberated in the formation of [Rh(CO)(S,N,S-
H2EtSNS)]2+ afforded the two chloridorhodates [Rh-
(CO)2Cl2]– and, due to the formation of a new coordination
site by the loss of a CO molecule, the H2EtSNS+ cation
could act as ligand for a Rh(CO)+ group. A more conve-
nient preparation of 7 was achieved by treating (H2EtSNS)-
Cl with [Rh(CO)2Cl]2 in a 2:3 ratio (Scheme 7). A view of
its structure, determined by X-ray diffraction methods, is
reported in Figure 7.
Scheme 8. Formation of complexes 8 and 9 by protonation of
[Rh(CO)EtSNS] and direct complexation of [Rh(CO)]+ species.
Scheme 7.
Figure 7. ORTEP plot of the molecular structure of 7. Thermal
ellipsoids are drawn at the 30% probability level. H atoms are omit-
ted for clarity. Symmetry transformation used to generate equiva-
lent atoms: 1 – x, y, 1/2 – z.
Figure 8. ORTEP plot of the molecular structure of 8·thf in 8·2thf.
Thermal ellipsoids are drawn at the 30% probability level. H atoms
are omitted for clarity. The second thf molecule was not involved
in interactions with the complex and is omitted for clarity.
The formation of 7 suggested that other stable proton-
ated derivatives of [Rh(CO)(EtSNS)] could be obtained
It is worthy to note that in 7, 8, and [Rh(CO)(EtSNS)],
(Scheme 8). Indeed, [Rh(CO)(S,N,SЈ-HEtSNS)]PF6 ([Rh- the coordination geometry is almost identical. The ligands
(CO)(EtSNS)]·HPF6, 8) was synthesized by protonation of correspond to, respectively, cationic H2EtSNS+, neutral–
[Rh(CO)(EtSNS)] with an equimolar amount of HPF6 or zwitterionic HEtSNS, and dianionic–cationic EtSNS–. In
by complexation of [Rh(CO)2(MeCN)2]PF6 with HEtSNS. the solid state infrared HATR spectra of [Rh(CO)(EtSNS)],
In a similar manner, [Rh(CO)(S,N,S-H2EtSNS)](PF6)2 8, and 9, the CO stretching frequency increases (1951, 1979,
([Rh(CO)(EtSNS)]·2HPF6, 9) was prepared by protonation and 1996 cm–1, respectively, in the solid state; 1968, 1994,
of [Rh(CO)(EtSNS)] with two equimolar amounts of HPF6 and 2017 cm–1, respectively, in CH2Cl2 solution), which
Eur. J. Inorg. Chem. 2008, 2302–2312
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2307

M. Delferro, D. Cauzzi, R. Pattacini, M. Tegoni, C. Graiff, A. Tiripicchio
FULL PAPER
suggests that the protonation of [Rh(CO)(EtSNS)] results tively) was calculated by DFT methods.[1a] A positive
in a decreased electron density on the metal center and thus charge is always shared by the two P atoms.
in a weaker RhǞCO back donation.[13] The P–N distances
in 7 and 8 are slightly shorter than those of [Rh(CO)-
Protonation Reaction of Complex 1
(EtSNS)], as well as the P–C ones. Because the protonation
of HEtSNS does not affect the P–N–P system (see Table 1),
this small change could result from variations in the ligating
properties of the thioamidyl/thioamidic groups. In fact, the
Rh–S bond length increases in the 7, 8, and [Rh(CO)-
(EtSNS)] series, which in turn affects the P–N–P system
through the metal center. The thioamidyl fragment, which
is more thiolato in character, is a stronger donor than the
thioamidic group of thione character. The Rh–N distance is
slightly shorter in the protonated species, which is probably
influenced by the lower electron density on the Rh atom.
The 31P{1H} NMR chemical shift is strongly affected by
the protonation: [Rh(CO)(EtSNS)] shows a sharp singlet at
δ = 16.6 ppm (CDCl3), whereas in 9 the two equivalent P
atoms resonate at δ = 38.2 ppm. The 31P{1H} NMR spec-
troscopic data recorded in CDCl3 for 8 show two sharp sin-
glets (35.0 and 19.9 ppm) assigned, respectively, to the P
atoms bound to the thioamidic and thioamidyl coordinated
functionalities,[14] which is suggestive of the absence of fast
proton exchange. The solid state 31P{1H} NMR-MAS spec-
The protonation behavior of complex 1 in the presence
of HCl or HOTf does not parallel that of [Rh(CO)EtSNS];
a change in the ligand coordination geometry was observed,
as depicted in Scheme 9. When one equivalent of HCl was
used, S-monodentate HEtSNS was formed, and the chlo-
ride anion substituted a sulfur atom in the coordination
sphere of Rh to yield complexes 4 and 4a. With two equiva-
lents of HCl, the H2EtSNS+ cation formed and the second
chloride anion entered into the coordination sphere of Rh
to yield complex 6. When HOTf was used, the formation
of 2 was observed, together with one equivalent of
H2EtSNS+. The different reactivity is apparently due to the
lower stability of the bidentate complex with respect to the
tridentate [Rh(CO)EtSNS].
Acid–Base Properties of [Rh(CO)(EtSNS)]
Protonation equilibria of [Rh(CO)(EtSNS)] were studied
trum of compound 8 was recorded and two peaks (36.1 and by means of spectroscopic UV/Vis titration in CH2Cl2 by
24.1 ppm) were observed. As a reference, in the solid-state using a solution of HOTf in a 1.5% mixture of methanol in
NMR spectrum of HEtSNS, two singlets are found at 8.1 dichloromethane.[15] Under these conditions it was assumed
(thioamidic) and 2.6 ppm (thioamidyl), which shows that that HOTf is completely dissociated and acts as a su-
Rh coordination exerts a strong influence on the chemical peracid[16] so that the concentration of H+ can be consid-
shift of the P atoms. The formal Lewis structures depicted ered equal to the analytical concentration of the acid added
in Scheme 8 are in agreement with this observation and (the methanol is present to allow the triflic acid to com-
with the bond lengths discussed above. The thioamidic pletely dissolve and to level HOTf to CH3OH2+).[16b] These
groups can be considered neutral in 8 and 9; the formal circumstances allow for the determination of the pKa in this
charge on the Rh atom is negative in [Rh(CO)(EtSNS)], solvent on an absolute scale. In the literature, protonation
neutral in 8, and positive in 9. The Mulliken atomic net equilibria of ligands and complexes in CH2Cl2 were studied
+
charge for complex [Rh(CO)(EtSNS)] (–0.150, –0.157, by means of proton exchange reactions by using HP(Cy)3
–0.177 in the gas phase, in cyclohexane, and in thf, respec- (Cy = cyclohexyl) as a weak acid, for which a pKa = 9.7
Scheme 9. Acid–base reactions for complex 1.
2308
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2302–2312

Determination of Absolute pKa Values of RhI Complexes
was established by literature convention.[17] This pKa scale, havior was observed for Pd complexes of thioamidic SNS
however, represents a relative pKa ladder that should be re- pincer ligands but protonation constants were not deter-
+
scaled to the real HP(Cy)3 acidity constant.[18] The em- mined.[19] Unfortunately, although the protonation equilib-
ployment of triflic acid as titrant, dissolved in CH2Cl2 with ria of several complexes were investigated in the last dec-
small amounts of methanol, allows direct evaluation of pro- ade[16,17] the dissociation constants were all referenced to
+
tonation equilibria in a similar way as studying the proton- the HP(Cy)3 standard, and no direct comparison with
ation equilibria in water by means of fully dissociated HCl. those values is possible. Titration of HEtSNS by UV/Vis
In the titration of [Rh(CO)(EtSNS)] with triflic acid, was not possible due to the absence of suitable variations
three very well-defined isosbestic points were detected: one in the absorption spectra. The titration was performed by
at 438 nm for acid to complex ratios from 0 to 1 and two monitoring the protonation by 31P NMR spectroscopy. Un-
at 509 and 385 nm for acid to complex ratios from 1 to 2 der these conditions, the basicity of the ligand is too high
(Figure 9). These results are in agreement with the expected for pKa determination.
diprotic nature of [Rh(CO)(H2EtSNS)]2+ and the presence
of two distinct protonation processes involving [Rh(CO)-
(EtSNS)].
Conclusions
The zwitterionic ligand HEtSNS is amphoteric and pos-
sesses ligating properties in all of its three forms: zwitter-
ionic–neutral, zwitterionic–anionic, and cationic. This
work, together with previous results, evidenced its geometri-
cal versatility, which allows the following coordination fash-
ions: S-monodentate (compound 4); eight-membered che-
lating S,S-bidentate with 180° bite-angle ([Au(EtSNS)][2]) or
90° bite-angle (compound 1); S,N,S-bis(chelating) ([Rh-
(CO)EtSNS],[1] [Cu(EtSNS)][2]); and N,N,N-tridentate co-
ordination in [Na(EtSNS)H2O]2, in which monodentate S-
coordination is also found. The S atoms are also able to
bridge as in clusters described in ref.[2] and in dinuclear
complex 2. In the case of [Rh(cod)]+ precursors, the coordi-
nation geometry and complex nuclearity in solution are
strongly dependent on the polarity of the solvent. Note-
worthy, HEtSNS, EtSNS–, and H2EtSNS+ show the same
S,N,S-coordinating properties towards the cationic metal
species [Rh(CO)]+ {compounds [Rh(CO)EtSNS], 7, 8, and
9, respectively). Protonated complexes can be prepared also
by reaction of acids with [Rh(CO)EtSNS], whose two pKa
have been determined in CH2Cl2 solution. The protonation
causes a change in the electron density on the Rh atom, as
evidenced by the stretching vibration of the coordinated CO
molecule, which could be exploited in tuning its catalytic
properties, for instance in reactions involving oxidative ad-
dition on the metal center.
Figure 9. UV/Vis spectra (300–600 nm) for the titration of [Rh(CO)-
(EtSNS)] with HOTf in CH2Cl2.
Two pKa values of 4.8(4) and 6.5(3) (σ = 4.11ϫ10–2)
were determined for the dissociation processes:
[Rh(CO)(H2EtSNS)]2+ h [Rh(CO)(HEtSNS)]+ + H+
[Rh(CO)(HEtSNS)]+ h [Rh(CO)(EtSNS)] + H+
A representative distribution diagram for the [Rh(CO)-
(H2EtSNS)]2+ complex is shown in Figure 10 {UV spectra
of pure samples of [Rh(CO)(EtSNS)], [Rh(CO)(EtSNS)]·
HOTf and [Rh(CO)(EtSNS)]·2HOTf are reported in the
Supporting Information}. Recently, a similar acid–base be-
Experimental Section
General Procedure and Materials: All manipulations were carried
out at room temperature in air or, when necessary, under a nitrogen
atmosphere by standard Schlenk techniques. tBuOK, EtNCS,
[Rh(cod)Cl]2, [Rh(CO)2Cl]2, AgPF6, AgOTf (OTf = CF3SO3),
HOTf, HPF6 (60% aqueous solution), and NaH were purchased
and used as received (Aldrich and Fluka). The solvents was dried
and distilled by standard techniques. Ph2PNHPPh2 (dppa),[20]
HEtSNS,[1] (H2EtSNS)Cl,[1] [Rh(CO)(EtSNS)],[2] [Rh(CH3CN)2-
–
(CO)2]X, and [Rh(thf)2(CO)2]X (X = PF6 , OTf–)[21] were prepared
as described elsewhere. Elemental analyses were carried out with a
Carlo Erba EA1108 microanalyzer. FTIR spectra (4000–400 cm–1)
were recorded with a Nicolet Nexus spectrophotometer equipped
1
with a Smart Orbit HATR accessory (diamond crystal). H NMR
Figure 10. Distribution diagram for the titration of [Rh(CO)-
(EtSNS)] (CRh = 1ϫ10–4 ).
(300.13 MHz, TMS) and 31P{1H} NMR (161.98 MHz, external
Eur. J. Inorg. Chem. 2008, 2302–2312
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2309

M. Delferro, D. Cauzzi, R. Pattacini, M. Tegoni, C. Graiff, A. Tiripicchio
FULL PAPER
reference 85% H3PO4) spectra were recorder with Bruker instru-
ments (AC300 Avance and AMX400, respectively) by using deuter-
ated solvents (CDCl3, CD2Cl2, [D6]DMSO, and CD3OD). The so-
lid-state 31P{1H}NMR-MAS spectra were recorded with a Bruker
Avance 300 MHz operating at a frequency of 121.57 MHz and
[{Rh(cod)}2(µ-S,S-EtSNS)]OTf (2): A solution of AgOTf (0.077 g,
0.30 mmol) in thf (10 mL) was added dropwise to a solution of
[Rh(cod)Cl]2 (0.075 g, 0.15 mmol) in thf (10 mL). The resulting re-
action mixture was stirred at room temperature for 10 min. Solid
AgCl was removed by filtration. To this solution was added com-
plex 1 (0.115 g, 0.15 mmol), and the purple reaction mixture was
stirred for 1 h. Evaporation of the volatiles yielded 2 as a purple
powder (Yield: 76%). The complex can be recrystallized by layering
1
equipped with a MAS 4 mm H-X probe. Sample rotation was set
at 1010 Hz. NH4H2PO4 was used as an external reference (δ =
–
0 ppm), and PF6 (–144 ppm) was used as an internal secondary
1
reference. A Micromass Quattro LC triple quadrupole instrument
equipped with an electrospray interface (Masslynx v. 3.4 software)
was used for MS data collection and processing. The nebulizing
gas (nitrogen, 99.999% purity) and the desolvation gas (nitrogen,
99.998% purity) were delivered at a flowrate of 80 and 500 Lh–1,
respectively. MS (ESI) analyses were performed by operating the
mass spectrometer in positive ion (PI) mode and by acquiring mass
spectra over the scan range m/z 100–2800 with a step size of 0.1 Da
and a scan time of 2.7 s. The interface operating parameters were:
source temperature 70 °C, desolvation temperature 70 °C, ESI(+)
capillary voltage 3.0 kV, cone voltage 15 V, rf lens 0.3 V. Absorp-
tion spectra in the range 300–600 nm were recorded with a Perkin–
Elmer Lambda 25 spectrophotometer by using matched quartz
cells of 1 cm path length and dichloromethane as reference.
hexane onto a thf solution to afford violet crystals of 2·1/2thf. H
NMR (300.13 MHz, CDCl3): δ = 7.7–7.2 (m, 20 H, Ph), 4.20 (dq,
4
3JH,H = 7.2 Hz, JH,P = 4.2 Hz, 4 H, CH2CH3), 4.23 (br., 4 H,
CH=CH, cod), 3.73 (s, 4 H, CH=CH, cod), 2.42 (br., 8 H, CHH,
3
cod), 1.95 (m, 8 H, CHH, cod), 1.45 (t, JH,H = 7.2 Hz, 6 H,
CH2CH3) ppm. 31P{1H} NMR (161.98 MHz, CDCl3): δ = 17.9 (s)
ppm. C47H54F3N3O3P2Rh2S3 (1129.90): calcd. C 49.96, H 4.82, N
3.72, S 8.51; found C 50.12, H 4.83, N 3.63, S 8.74. MS (ESI+,
CH2Cl2): m/z (%) = 980.26 (100) [M]+.
Reaction of
1 with [Rh(cod)Cl]2 To Form [{Rh(cod)}2(µ-S,S-
EtSNS)]Cl (3): A solution of [Rh(cod)Cl]2 (0.024 g, 0.097 mmol) in
CH2Cl2 (10 mL) was added to a solution of 1 (0.150 g, 0.195 mmol)
in CH2Cl2 (10 mL). The resulting orange reaction mixture was
stirred for 10 min. Methanol (40 mL) was added, and the solution
color turned to purple. Evaporation of the solvent yielded the start-
ing mixture of the reagents. The same reaction was performed in
CD3OD in order to obtain NMR spectroscopic data for 3. 1H
NMR (300.13 MHz, CD3OD): δ = 7.5–7.2 (m, 20 H, Ph), 4.21 (m,
4 H, CH2CH3, m, 4 H, CH=CH, cod), 3.7 (s, 4 H, CH=CH, cod),
2.40 (s, 8 H, CHH, cod), 1.95 (m, 8 H, CHH, cod), 1.45 (t, 6 H,
CH2CH3) ppm. 31P{1H} NMR (161.98 MHz, CD3OD): δ = 17.1
[Na(EtSNS)]: A suspension of NaH (0.043 g, 1.79 mmol) in thf
(20 mL) was added to a solution of HEtSNS (0.500 g, 0.89 mmol)
in thf (20 mL) under an atmosphere of nitrogen. The resulting sus-
pension was stirred for 1 h. Unreacted NaH was removed by fil-
tration. Evaporation of the solvent afforded [Na(EtSNS)] as a white
powder (Yield: 97%). [Na(EtSNS)] was recrystallized by slow evap-
oration of a CH2Cl2/hexane solution to yielding [Na(EtSNS)-
H2O]2. 1H NMR (300.13 MHz, [D6]DMSO): δ = 8.0–7.2 (m, 20 H,
(s) ppm. MS (ESI+, MeOH): m/z (%)
= 980.26 (100)
3
4
Ph), 3.42 (qd, JH,H = 7.2 Hz, JH,P = 5.0 Hz, 4 H, CH2CH3), 0.99
(t, JH,H
[{Rh(cod)}2(µ-S,S-EtSNS)]+.
= 7.2 Hz, 6
H, CH2CH3) ppm. 31P{1H} NMR
3
[Rh(cod)(S-HEtSNS)Cl] (4): A solution of HEtSNS (0.145 g,
0.26 mmol) in CH2Cl2 (20 mL) was added to a solution of [Rh(cod)-
Cl]2 (0.064 g, 0.13 mmol) in CH2Cl2 (10 mL). The red reaction mix-
ture was stirred for 10 min. Evaporation of the solvent yielded 4 as
a yellow powder (Yield: 92%), which could be recrystallized by
layering hexane onto a CH2Cl2 solution. In solution, compound 4
was found to be in equilibrium with [Rh(cod)(S,SЈ-HEtSNS)]Cl
(161.98 MHz, [D6]DMSO): δ = 7.2 (s) ppm. C30H30N3NaP2S2
(581.65): calcd. C 61.95, H 5.20, N 7.22, S 11.03; found C 62.31,
H 5.31, N 7.25, S 10.87.
(H2EtSNS)PF6: Solid HEtSNS (0.073 g, 0.13 mmol) was dissolved
in CH2Cl2 (10 mL), which resulted in a yellow solution; a solution
of HPF6 (60% in H2O, 0.2 mL, 2.4 mmol) was added, and the reac-
tion mixture was vigorously shaken. The aqueous phase was re-
moved, and the resulting organic phase was dried with anhydrous
Na2SO4 and evaporated under reduced pressure to obtain
(H2EtSNS)PF6 as a yellow powder (Yield: 98%). 1H NMR
(300.13 MHz, CDCl3): δ = 12.8 (s, 2 H, N-H), 8.0–7.3 (m, 40 H,
1
(4a). Data for 4: H NMR (300.13 MHz, CDCl3): δ = 11.37 (br. s,
N-H), 7.7–7.2 (m, Ph), 4.15 (br. m, CH=CH, cod), 4.00 (br. m,
CH2CH3), 3.93 (br. m, CH2CH3), 3.63 (br. m, CH=CH, cod), 2.13
3
(br. s, CHH, cod), 1.48 (m, CHH, cod), 1.27 (t, JH,H = 6 Hz,
CH2CH3) ppm. signals of compound 4a (except for NH group)
were underimposed to signals of compound 4 thus affecting peak
integration.31P{1H} NMR (161.98 MHz, CDCl3): δ = 11.1 (d, 2JP,P
3
4
Ph), 4.05 (qd, JH,H = 7.2 Hz, JH,P = 5.0 Hz, 4 H, CH2CH3), 1.41
(t, JH,H = 7.2 Hz, CH2CH3) ppm. 31P{1H} NMR (161.98 MHz,
3
CDCl3): δ = 13.3 (s) ppm. C30H32F6N3P3S2 (705.64): calcd. C
51.06, H 4.57, N 5.95, S 9.09; found C 52.03, H 4.62, N 6.15, S
9.54.
2
= 15.9 Hz), 10.5 (d, JP,P = 15.9 Hz) ppm. C38H43ClN3P2RhS2
(806.21): calcd. C 56.61, H 5.38, N 5.21, S 7.95; found C 57.46, H
5.46, N 5.37, S 8.10. Data for 4a: 31P{1H} NMR (161.98 MHz,
CDCl3): δ = 12.4 (s, integral ratio with respect to complex 4 = 1:10)
ppm. 1H NMR (300.13 MHz, CDCl3): δ = 12.82 (br. s, N-H) ppm.
[Rh(cod)(S,S-EtSNS)] (1): Solid [Rh(cod)Cl]2 (0.100 g, 0.203 mmol)
and HEtSNS (0.226 g, 0.406 mmol) were dissolved in CH2Cl2
(10 mL). A solution of tBuOK (0.046 g, 0.406 mmol) in MeOH
(2 mL) was then added, and the reaction mixture was stirred for
30 min. The volatiles were evaporated, and the resulting yellow
powder was redissolved in CH2Cl2 (5 mL). KCl was removed by
filtration. Evaporation of the solvent afforded 1 as a yellow powder
Reaction of [Rh(CO)2Cl]2 with HEtSNS: A solution of HEtSNS
(0.280 g, 0.50 mmol) in CH2Cl2 (10 mL) was added under an atmo-
sphere of nitrogen to [Rh(CO)2Cl]2 (0.097 g, 0.25 mmol) dissolved
in CH2Cl2 (15 mL). The solution color changed rapidly from yellow
to red. Stirring was continued for 2 h, and the volatiles were re-
(Yield: 85%). Complex 1 could be recrystallized by layering hexane
1
onto a CH2Cl2 solution. H NMR (300.13 MHz, CDCl3): δ = 7.8– moved under vacuum to afford a red-orange powder. Three com-
7.2 (m, 20 H, Ph), 3.9–3.7 (m, 4 H, CH=CH, cod; m, 4 H,
pounds {[Rh(CO)2(S-EtSNS)Cl], [H2EtSNS][Rh(CO)2Cl2] (5), and
CH2CH3), 1.87 (br., 4 H, CHH, cod), 1.52 (m, 4 H, CHH, cod),
[Rh(CO)(EtSNS)])} were spectroscopically identified as the major
3
1.18 (t, JH,H = 7.1 Hz, 6 H, CH2CH3) ppm. 31P{1H} NMR components of the crude product. Crystals of compound 5 were
(161.98 MHz, CDCl3): δ = 9.7 (s) ppm. C38H42N3P2RhS2 (769,74):
calcd. C 59.29, H 5.50, N 5.46, S 8.33; found C 59.11, H 5.64, N
5.29, S 8.16.
obtained by layering hexane onto a solution of the crude product.
31P{1H} NMR (161.98 MHz, CDCl3): δ = 16.85 (s, integration
0.75, [Rh(CO)(EtSNS)]), 13.30 (s, integration 1, compound 5),
2310
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2302–2312

Determination of Absolute pKa Values of RhI Complexes
2
2
12.90 (d, JP,P = 15.2 Hz, integration 0,12), 11.44 (d, JP,P
=
[Rh(CO)(H2EtSNS)]2+. FTIR (Diamond crystal HATR): ν = 1996
˜
15.2 Hz, integration 0.12, [Rh(CO)2(S-EtSNS)Cl]) ppm. FTIR
(νCO) cm–1. FTIR (CH Cl ): ν = 2017 (νCO) cm–1.
˜
2
2
(Diamond crystal HATR): ν = 2071 (νCO), 1992 (νCO) cm–1.
˜
[Rh(CO)(S,N,S-HEtSNS)]OTf: A solution of [Rh(CH3CN)2(CO)2]-
OTf (0.200 g, 0.37 mmol) in CH2Cl2 (10 mL) was added under an
[H2EtSNS][Rh(cod)Cl2] (6): Solid (H2EtSNS)Cl (0.242 g,
0.406 mmol) was added to a solution of [Rh(cod)Cl]2 (0.100 g,
0.203 mmol) in CH2Cl2 (20 mL). The resulting orange reaction
mixture was stirred for 10 min. Evaporation of the volatiles af-
forded [H2EtSNS][Rh(cod)Cl2] (6) as an orange powder (Yield:
92%). The salt can be recrystallized by layering hexane onto its
CH2Cl2 solution. 1H NMR (300.13 MHz, CDCl3): δ = 12.70 (br.
s, 2 H, N-H), 7.6–7.1 (m, 20 H, Ph), 4.18 (br. m, 8 H, CH2CH3
and CH=CH), 2.48 (br. m, 4 H, CHH, cod), 1.73 (br. m, 4 H,
atmosphere of nitrogen to
a solution of HEtSNS (0.215 g,
0.37 mmol) in CH2Cl2 (15 mL). Stirring was continued for 2 h, and
the solution color turned to red. Evaporation of the volatiles af-
forded compound the product as an orange microcrystalline pow-
der (Yield: 87%). 1H NMR (300.13 MHz, [D6]DMSO): δ = 11.0
3
(br. s, 1 H, N-H), 7.7–7.4 (m, 20 H, Ph), 3.67 (m, JH,H = 6.3 Hz,
3
4 H, CH2CH3), 1.16 (t, JH,H = 6.3 Hz, 6 H, CH2CH3) ppm.
31P{1H} NMR (161.98 MHz, CDCl3): δ = 35.0 (s), 19.9 (s) ppm.
C32H31F3N3O4P2RhS3 (839.65): calcd. C 45.77, H 3.72, N 5.00, S
11.45; found C 45.62, H 3.69, N 4.97, S 11.39. MS (ESI+, MeOH):
m/z (%) = 691.6 (100) [Rh(CO)(HEtSNS)]+. FTIR (Diamond crys-
3
CHH, cod), 1.49 (t, JH,H = 7.05 Hz, 6 H, CH2CH3) ppm.
31P{1H} NMR (161.98 MHz, CDCl3):
δ = 13.6 (s) ppm.
C38H44Cl2N3P2RhS2 (842,67): calcd. C 54.16, H 5.26, N 4.99, S
7.61; found C 54.49, H 5.18, N 5.01, S 7.73.
tal HATR): ν = 1979 (νCO) cm–1.
˜
[Rh(CO)(S,N,S-H2EtSNS)](OTf)2: In a Schlenk tube and under an
atmosphere of nitrogen, a solution of [Rh(CO)(EtSNS)] (0.200 g,
0.29 mmol) in chloroform (10 mL) was treated with water solution
of HOTf (10 mL) in large excess with respect to the rhodium pre-
cursor. The yellow solution turned orange-red and a precipitate
formed, which was separated by decanting. The two liquid phases
were separated, and the organic layer was dried to give an orange-
red solid. The two solids were recrystallized from hot methanol to
afford the product as an orange microcrystalline powder (Yield:
90%). 1H NMR (300.13 MHz, CDCl3): δ = 11.1 (br. s, 2 H,
[Rh(CO)(S,N,S-H2EtSNS)][Rh(CO)2Cl2]2 (7):
A
solution of
(H2EtSNS)Cl (0.100 g, 0.17 mmol) in CH2Cl2 (10 mL) was added
under an atmosphere of nitrogen to a solution of [Rh(CO)2Cl]2
(0.099 g, 0.255 mmol) in CH2Cl2 (10 mL). Stirring was continued
for 2 h, and the solution color turned to dark red. Evaporation of
the volatiles afforded 7 as a red microcrystalline powder (Yield:
1
91%). H NMR (300.13 MHz, [D6]DMSO): δ = 11.08 (br. s, 2 H,
3
4
N-H), 7.9–7.3 (m, 20 H, Ph), 4.15 (dd, JH,H = 6.3 Hz, JH,P
=
3
6.3 Hz, 4 H, CH2CH3), 1.25 (t, JH,H = 6.3 Hz, 6 H, CH2CH3)
ppm. 31P{1H} NMR (161.98 MHz, CDCl3): δ = 38.2 (s) ppm.
C35H32Cl4N3O5P2Rh3S2 (1151.26): calcd. C 36.51, H 2.80, N 3.65,
S 5.57; found C 36.39, H 2.86, N 3.54, S 5.71. MS (ESI+, MeOH):
m/z (%) = 691.6 (100) [Rh(CO)(H2EtSNS)]2+. FTIR (Diamond
3
N-H), 7.8–7.4 (m, 20 H, Ph), 4.15 (m, JH,H = 6.3 Hz, 4 H,
3
CH2CH3), 1.18 (t, JH,H = 6.3 Hz, 6 H, CH2CH3) ppm. 31P{1H}
NMR (161.98 MHz, [D6]DMSO):
δ
=
38.2 (s) ppm.
C33H32F6N3O7P2RhS4 (989.73): calcd. C 40.04, H 3.26, N 4.24, S
12.96; found C 39.97, H 3.23, N 4.19, S 12.89. MS (ESI+, MeOH):
m/z (%) = 691.6 (100) [Rh(CO)(H2EtSNS)]2+. FTIR (Diamond
crystal HATR): ν = 2064 (m, νCO), 2011 (m, νCO), 1990 (s, νCO),
˜
1985 (s, νCO), 1961 (m, sh. νCO) cm–1. FTIR (CH Cl ): ν = 2074
˜
2
2
(s, νCO[Cl2Rh(CO)2]–), 1998 (s, νCO[Cl2Rh(CO)2]–), 2014 (m, sh,
crystal HATR): ν = 1995 (νCO) cm–1.
˜
νCORh–CO) cm–1.
UV/Vis Titration: The spectra of reference complexes [Rh(CO)-
(EtSNS)], [Rh(CO)(HEtSNS)]OTf, and [Rh(CO)(H2EtSNS)]-
(OTf)2 were recorded in 1ϫ10–3  dichloromethane solutions (see
Figures S4, S5, and S6, respectively; Supporting Information). Pu-
rity was previously checked by 1H and 31P{1H} NMR spectroscopy
and elemental analysis. A dichloromethane stock solution of
[Rh(CO)(EtSNS)] (7ϫ10–3 ) was prepared by weight and diluted
to 1ϫ10–4  for the UV/Vis titration. KOH aqueous solutions (ca.
0.2 ) were prepared by diluting concentrated Merck Titrisol am-
poules and standardized with the reported procedures.[22] The ti-
trant solution of HOTf (ca. 15ϫ10–3 ) was prepared by dissolving
pure triflic acid (15 µL) in CH2Cl2 (10 mL) with the addition of
reagent grade methanol (150 µL). Its titre was determined by titrat-
ing a suspension of a dichloromethane solution in water (2 mL)
with the standard KOH solution using phenolphthalein as indi-
cator. Thirty UV/Vis spectra were collected at different acid/com-
plex ratios (range between 0 and 2.2) by the addition of titrant
[Rh(CO)(S,N,SЈ-HEtSNS)PF6 (8): A solution of HEtSNS (0.280 g,
0.50 mmol) in CH2Cl2 (10 mL) was added under an atmosphere of
nitrogen to
a solution of [Rh(CH3CN)2(CO)2]PF6 (0.193 g,
0.50 mmol) in CH2Cl2 (10 mL). Stirring was continued for 2 h, and
the solution color turned to violet red. Evaporation of the volatiles
afforded 8 as a red microcrystalline powder (Yield: 89%). The com-
plex can be recrystallized by layering pentane onto a thf solution
of 8 to afford red crystals of 8·2thf. 1H NMR (300.13 MHz,
[D6]DMSO): δ = 11.0 (br. s, 1 H, N-H), 7.7–7.4 (m, 20 H, Ph), 3.67
3
3
(m, JH,H = 6.2 Hz, 4 H, CH2CH3), 1.16 (t, JH,H = 6.2 Hz, 6 H,
CH2CH3) ppm. 31P{1H} NMR (161.98 MHz, CDCl3): δ = 35.0 (s),
19.9 (s) ppm. C31H31F6N3OP3RhS2 (835.55): calcd. C 44.56, H
3.74, N 5.03, S 7.67; found C 44.45, H 3.69, N 4.96, S 7.58. MS
(ESI+, MeOH): m/z (%) = 690.6 (100) [Rh(CO)(HEtSNS)]+. FTIR
(Diamond crystal HATR): ν = 1979 (νCO) cm–1. FTIR (CH2Cl2):
˜
ν = 1994 (νCO) cm–1.
˜
(25 µL) at each step {concentration of [Rh(CO)(EtSNS)]
=
[Rh(CO)(S,N,S-H2EtSNS)](PF6)2 (9): A solution of [Rh(CH3CN)2-
(CO)2]PF6 (0.193 g, 0.50 mmol) in CH2Cl2 (10 mL) was added un-
der an atmosphere of nitrogen to a solution of (H2EtSNS)PF6
(0.352 g, 0.50 mmol) in CH2Cl2 (15 mL). Stirring was continued for
2 h, and the solution color turned to red. Evaporation of the vola-
tiles afforded 9 as an orange microcrystalline powder (Yield: 88%).
1H NMR (300.13 MHz, [D6]DMSO): δ = 11.1 (br. s, 2 H, N-H),
1ϫ10–4 , V0 = 50 mL}. The spectroscopic data were processed to
calculate the protonation constants (pKa) by means of the program
SPECFIT 32 by using the molar absorbance values of the pure
protonated complexes as fixed parameters.[23] The wavelength range
taken into account in the calculations was limited to 380–600 nm,
where the complexes showed major variations in their absorption
spectra. The distribution diagram was calculated and plotted using
the program HYSS.[24]
3
7.8–7.4 (m, 20 H, Ph), 4.15 (m, JH,H = 6.2 Hz, 4 H, CH2CH3),
3
1.18 (t, JH,H = 6.2 Hz, 6 H, CH2CH3) ppm. 31P{1H} NMR
(161.98 MHz, CDCl3): δ = 38.2 (s) ppm. C31H32F12N3OP4RhS2 X-ray Data Collection, Structure Solution, and Refinement: The in-
(981.52): calcd. C 37.93, H 3.29, N 4.28, S 6.53; found C 37.55, H tensity data of all compounds were collected at room temperature
3.21, N 4.15, S 6.38. MS (ESI+, MeOH): m/z (%) = 691.6 (100) with a Bruker AXS Smart 1000[25] single-crystal diffractometer
Eur. J. Inorg. Chem. 2008, 2302–2312
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2311

M. Delferro, D. Cauzzi, R. Pattacini, M. Tegoni, C. Graiff, A. Tiripicchio
FULL PAPER
[5] Differently from CH2Cl2, the solvent used for the crystalli-
zation, it is known that CHCl3 may contain amounts of HCl
produced by exposure to light and oxygen.D. G. Hill, J. Am.
Chem. Soc. 1932, 54, 32–40.
[6] a) B. W. Maxey, A. I. Popov, J. Inorg. Nucleic Chem. 1970, 32,
1029–1032; b) L. Matilainen, M. Leskelae, M. Klinga, J. Chem.
Soc., Chem. Commun. 1995, 421–422.
equipped with an area detector by using a graphite monochro-
mated Mo-Kα radiation (λ = 0.71073 Å). Crystallographic and ex-
perimental details of the structures are summarized in Table S1
(Supporting Information). When necessary, an empirical correction
for absorption was made. The structures were solved by Patterson
and Fourier methods and refined by full-matrix least-squares pro-
2 [26]
cedures (based on Fo )
first with isotropic thermal parameters
[7]
The overall reactivity towards [Rh(cod)Cl]2, solvent-dependent
equilibria, and the reaction of complex 1 with CO are depicted
in Schemes S1 and S2 (Supporting Information).
and then with anisotropic thermal parameters in the last cycles of
refinement for all the non-hydrogen atoms. The hydrogen atoms
were introduced into the geometrically calculated positions and re-
fined riding on the corresponding parent atoms except for the am-
idic hydrogen atoms and those of the water molecule in [Na-
(EtSNS)H2O]2, which were localized in the 2Fo – Fc map and re-
fined isotropically. In (H2EtSNS)PF6 one of the two ethyl groups
[8] The mean value between 180 and 90°(respectively, found in
[Au(EtSNS)] and in 1) is 135° as found in [Ag(EtSNS)]2.
[9]
[10]
R. Chauvin, Eur. J. Inorg. Chem. 2000, 4, 577–591.
For recent examples, see: a) V. Miranda-Soto, J. J. Perez-Tor-
rente, L. A. Oro, F. J. Lahoz, M. L. Martin, M. Parra-Hake,
D. B. Grotjahn, Organometallics 2006, 25, 4374–4390; b) S.
Cai, G. X. Jin, Organometallics 2005, 24, 5280–5286; c) J. A.
Camerano, M. A. Casado, M. A. Ciriano, C. Tejel, L. A. Oro,
Dalton Trans. 2005, 18, 3092–3100.
–
and the PF6 anion were found disordered in two positions; in 2·
1/2thf the thf molecule was found disordered around the twofold
–
axis and in 8·2thf the PF6 anion was found disordered in two
positions.
[11]
[12]
[13]
V. Amendola, M. Bonizzoni, D. Esteban-Gomez, L. Fabbrizzi,
M. Licchelli, F. Sancenon, A. Taglietti, Coord. Chem. Rev.
2006, 250, 1451–1470.
a) V. L. Goedken, M. L. Vallarino, J. V. Quagliano, Inorg.
Chem. 1971, 10, 2682–2685; b) D. D. J. W. Mercer, H. A. Jenk-
ins, Inorg. Chim. Acta 2007, 360, 3091–3098.
S. C. Van der Slot, J. Duran, J. Luten, C. Paul, J. Kamer,
P. W. N. M. Van Leeuwen, Organometallics 2002, 21, 3873–
3883.
CCDC-671055 (for H2EtSNSPF6), -671056 {for [Na(EtSNS)-
H2O]2}, -671048 (for 1), -671049 (for 2), -671050 (for 4), -671051
(for 5), -671052 (for 6), -671053 (for 7), and -671054 (for 8), contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Center via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): An overall reactivity scheme for complexes derived from
Rh(cod) species; 31P{1H} NMR spectra in CDCl3 solution of the
reaction [Rh(cod)Cl]2 + 2HEtSNS and the reaction [Rh(CO)2-
Cl]2 + 2HEtSNS; UV/Vis spectra of compounds [Rh(CO)(EtSNS)],
[Rh(CO)(HEtSNS)]OTf, and [Rh(CO)(H2EtSNS)](OTf)2; crystal-
lographic and experimental details.
[14]
[15]
L. Boubekeur, S. Ulmer, L. Ricard, N. Mézailles, P. Le Floch,
Organometallics 2006, 25, 315–317.
During the titration, 30 additions of 25 µL of HOTf dissolved
in CH2Cl2/CH3OH were made to 50 mL of solution of
[Rh(CO)EtSNS] in neat dichloromethane. Under these condi-
tions, the final quantity of methanol in the titrated solution is
negligible (0.02%).
[16]
[17]
a) T. Fujinaga, I. Sakamoto, J. Electroanal. Chem. 1977, 85,
185–201; b) J. Bessiere, P. Gagne, Teintex 1978, 43, 5–12; c)
R. L. Benoit, D. Figeys, Can. J. Chem. 1991, 69, 1985–1988.
a) C. A. Streuli, Anal. Chem. 1960, 32, 985–987; b) G. Jia,
R. H. Morris, J. Am. Chem. Soc. 1991, 113, 875–883; c) R. H.
Morris, Chem. Eur. J. 2007, 13, 3796–3803.
T. Li, A. J. Lough, C. Zuccaccia, A. Macchioni, R. H. Morris,
Can. J. Chem. 2006, 84, 164–175.
R. A. Begum, D. Powell, K. Bowman-James, Inorg. Chem.
2006, 45, 964–966.
P. Bhattacharyya, J. D. Woollins, Polyhedron 1995, 14, 3367–
3388.
J. A. Osborn, R. Schrock, J. Am. Chem. Soc. 1971, 93, 3089–
3091.
F. Dallavalle, G. Folesani, R. Marchelli, G. Galaverna, Helv.
Chim. Acta 1994, 77, 1623–1630.
R. A. Binstead, B. Jung, A. D. Zuberbühler, SPECFIT Global
Analysis System, Version 3.0, Spectrum Software Associates,
Marlborough (MA, U.S.A.), 2004.
L. Alderighi, P. Gans, A. Ienco, D. Peters, A. Sabatini, A.
Vacca, Coord. Chem. Rev. 1999, 184, 311–318.
SMART Software Users Guide, Version 5.1, Bruker Analytical
X-ray Systems, Madison, WI, 1999.
a) SAINT Software Users Guide, Version 6.0, Bruker Analytical
X-ray Systems, Madison, WI, 1999; b) G. M. Sheldrick, SAD-
ABS, Bruker Analytical X-ray Systems, Madison, WI, 1999; c)
G. M. Sheldrick, SHELXL-97, Program for crystal structure
refinement; University of Göttingen, Germany, 1997.
Received: January 22, 2008
Acknowledgments
We thank the Università di Parma for funding this research. We
also thank Dr. Pierre Braunstein (Laboratoire de Chimie de Coor-
dination, CNRS Université Louis Pasteur, Strasbourg, France) for
access to the NMR spectroscopic facilities of the ULP and Dr.
Lionel Allouche (Service Commun de RMN, Université Louis
Pasteur, Strasbourg, France) for colleting the MAS-NMR spectra.
[18]
[19]
[20]
[21]
[22]
[23]
[1] a) M. Asti, R. Cammi, D. Cauzzi, C. Graiff, R. Pattacini, G.
Predieri, A. Stercoli, A. Tiripicchio, Chem. Eur. J. 2005, 11,
3413–3419; b) [Rh(CO)(EtSNS)]: 1H NMR (300 MHz, CDCl3,
3
25 °C, TMS): δ = 7.48–7.26 (m, 20 H, Ph), 3.75 (qd, JH,H
=
4
3
7.2 Hz, JH,P = 3.9 Hz, 4 H, CH2), 1.19 (t, JH,H = 7.2 Hz, 6
H, CH3) ppm. 31P{1H} NMR (400 MHz, CDCl3, 25 °C): δ =
16.6 ppm; c) FTIR (Diamond crystal HATR): ν = 1951 (ν
)
˜
CO
cm–1.
[24]
[25]
[26]
[2] R. Pattacini, L. Barbieri, A. Stercoli, D. Cauzzi, C. Graiff, M.
Lanfranchi, A. Tiripicchio, L. Elviri, J. Am. Chem. Soc. 2006,
128, 866–876.
[3] In more than 1000 structures of metal compounds containing
the PPN+ cation found in the Cambridge Crystallographic
Data Base, the shortest distance (4.047 Å) is reported in: J.
Chiba, K. Tanaka, Y. Ohshiro, R. Miyake, S. Hiraoka, M.
Shiro, M. Shionoya, J. Org. Chem. 2003, 68, 331–338.
[4] a) U. Englich, S. Chadwick, K. Ruhlandt-Senge, Inorg. Chem.
1998, 37, 283–293; b) M. Niemeyer, P. P. Power, Inorg. Chem.
1996, 35, 7264–7272.
Published Online: April 4, 2008
2312
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2302–2312