N,C "rollover" cyclometalated platinum(II) hydrides: Mono- and polynuclear derivatives

Article abstract of DOI:10.1021/om800027g

N,C cyclometalated platinum(II) hydrides were obtained by reaction of the "rollover" derivatives [Pt(N′-C(3))(Cl)(L)] (N-CH= 6-tert-butyl- and 6-phenyl-2,2′-bipyridine; L = DMSO, 3,5-Me₂-pyridine, PPh₃) with Na[BH₄]. Mono-

Full text of DOI:10.1021/om800027g

Organometallics 2008, 27, 3415–3421
3415
N,C “Rollover” Cyclometalated Platinum(II) Hydrides: Mono- and
Polynuclear Derivatives
Giovanni Minghetti, Sergio Stoccoro, Maria Agostina Cinellu, Giacomo Luigi Petretto, and
Antonio Zucca*
Dipartimento di Chimica, UniVersita` di Sassari, Via Vienna 2, 07100 Sassari, Italy
ReceiVed January 11, 2008
N,C cyclometalated platinum(II) hydrides were obtained by reaction of the “rollover” derivatives
[Pt(N′-C(3))(Cl)(L)] (N-CH) 6-tert-butyl- and 6-phenyl-2,2′-bipyridine; L ) DMSO, 3,5-Me2-pyridine,
PPh3) with Na[BH4]. Mono- or polynuclear hydrides, [Pt(N′-C(3))(H)(L)] or [Pt(N′-C(3))(µ-H)]n, are
formed depending on the nature of the neutral ligand, L. The unsaturated 14-electron fragments
[Pt(N′-C(3))(µ-H)] assemble to form oligomers with bridging hydrides. Spectroscopic characterization
provides evidence for tetramers both in solution (1H NMR) and in vapor phase (ESI-MS). In the tetrameric
systems the 2e-3c Pt-H-Pt bond is cleaved by π-acceptor ligands such as Ph3P and CO to give stable
mononuclear hydrides.
analogous platinum(IV) hydrides8 due to the interest of diimine
and diamine platinum(II) derivatives as models for the C-H
bond activation step in selective oxidation of hydrocarbons9 and
as being effective homogeneous catalysts for methane oxidation
by sulfuric acid.10
Introduction
Nitrogen ligands have never been popular in the chemistry
of transition metal hydrido complexes, the ancillary ligands most
frequently being π-acceptors, such as CO, Cp, and phosphorus
donors.1 As for the late transition metals, hydrides with nitrogen
ligands, with a few exception, are rare.
Platinum(II) hydrides containing cyclometalated rings arising
from activation of nitrogen ligands, despite the great number
of known systems, are even more rare: a perusal of the literature
provides only few examples that comprise both terminal and
bridging species. A dinuclear complex stabilized by a N-C-N
pincer ligand, [(N-C-N)Pt(µ-H)Pt(N-C-N)]+, H trans to C,
was described several years ago:11 the related terminal hydride,
[Pt(N-C-N)(H)], was not isolated. Another dinuclear hydride,
with terminal Pt-H bonds, [(H)Pt(PPh3)2(C10N8)(PPh3)2Pt(H)]
(C10N8H2 ) 4,4′,5,5′-tetracyano-2,2′-biimidazolo), was reported
In the particular case of platinum, a rich hydrido chemistry
has been developed since the first report in the late 1950s;2
nowadays an array of species are known in different oxidation
states, mainly platinum(II) and platinum(IV), but also a few
dinuclear, diamagnetic, platinum(I) compounds.3
Platinum(II) hydrides that contain metal-nitrogen bonds are
rare, and most of them, in addition to nitrogen donors, also
contain the more common classical phosphine ligands.4 Com-
pounds bearing only nitrogen ligands are few in number: they
include a cationic species with a neutral diamino ligand,
[Pt(dC(CH3)OCH2CH2)(tmeda)H]+ (tmeda ) N,N,N′,N′-tet-
ramethylethylenediamine),5 trispyrazolylborato mono- and di-
nuclear species,6 and some recent olefin hydrido complexes.7
In contrast, in recent years, much attention has been paid to the
(4) For example: (a) Gavrilova, I. V.; Gel’fman, M. I.; Razumosvskii,
V. V. Russ. J. Inorg. Chem. (Engl. Transl.) 1974, 19, 1360trans-
[Pt(H)(NH3)(PPh3)2]+: (b) Cowan, R. L.; Trogler, W. C. Organometallics
1987, 6, 2451 trans-[Pt(H)(NHPh)(PEt3)2]: (c) Park, S.; Roundhill, D. M.;
Rheingold, A. L. Inorg. Chem. 1987, 26, 3974 trans-[Pt(H)(NH2)(PR3)2],
[Pt(H)(µ-NH2)(PR3)]2: (d) Chang, X.; Lee, K.-E.; Kim, Y.-J.; Lee, S. W.
Inorg. Chim. Acta 2006, 359, 4436. trans-[Pt(H)(N3)(PEt3)2]: (e) Nelson,
J. H.; Schmitt, D. L.; Henry, R. A.; Moore, D. W.; Jonassen, H. B. Inorg.
Chem. 1970, 9, 2678. trans- [Pt(H)(tetrazolato)(PPh3)2]: (f) Roundhill, D. M.
Inorg. Chem. 1970, 9, 254 trans-[Pt(H)(imide)(PPh)2]: (g) Garcia, J. J.;
Casado, A. L.; Iretskii, A.; Adams, M.; Maitlis, P. M. J. Organomet. Chem.
1998, 558, 189–192 trans-[Pt(H)(C12H8N)(PR3)2] (C12H9N ) carbazolo):
(h) Chantson, J. T.; Lotz, S. J. Organomet. Chem., 2004, 689, 1315–1324
trans-[Pt(H)((1-azolyl)(PEt3)2] (i) Poverenov, E.; Gandelman, M.; Shimon,
L. J. W.; Rozenberg, H.; Ben-David, Y.; Milstein, D. Chem.-Eur. J. 2004,
10, 4673. (j) Grotjahn, D. B.; Gong, Y.; Di Pasquale, A. G.; Zakharov,
L. N.; Rheingold, A. L. Organometallics 2006, 25, 5693–5695.
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1997, 119, 848–849.
* To whom correspondence should be addressed. E-mail: zucca@uniss.it.
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R. H. Coord. Chem. ReV. 1985, 65, 1. (c) Recent AdVances in Hydride
Chemistry; Peruzzini, M., Poli, R., Eds.; Elsevier: New York, 2001.
(2) (a) Chatt, J.; Duncanson, L. A.; Shaw, B. L. Proc. Chem. Soc. 1957,
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10.1021/om800027g CCC: $40.75
2008 American Chemical Society
Publication on Web 06/21/2008

3416 Organometallics, Vol. 27, No. 14, 2008
Minghetti et al.
by Rasmussen et al.12 Mono- and dinuclear derivatives with
tridentateN-N-Cligands,[Pt(N-N-C)(H)]and[(N-N-C)Pt(µ-
H)Pt(N-N-C]+, H trans to N (N-N-CH ) 6-substituted-
2,2′-bipyridines), have been preliminarily reported13 and very
recently described in detail by us.14 The series includes species
arising from activation of C-H bonds of alkyl, benzyl, and
phenyl substituents, to give both [5,5] and [5,6] fused cyclo-
metalated rings. Spectroscopic evidence for a similar mono-
nuclear hydride has been given by Song and Morris.15 Two other
dinuclear complexes with terminal Pt-H bonds, where the metal
atoms are bridged by a 2-fold deprotonated 2,2′:6′,2′′-terpyri-
dine, [Pt2(terpy-2H)(H)2(L)2], have also been described.16
6-tert-butyl-2,2′-bipy, 1, and 6-phenyl-2,2′-bipy, 2,18 respec-
tively, were chosen. Throughout this paper we can anticipate
that the behavior of 1a (alkyl-substituted ligand) and 2a (aryl-
substituted ligand), as for the reactivity here discussed, is the
same. Compounds 1a and 2a were prepared, as previously
reported,17b according to reaction 1:
The N-N-C cyclometalated hydrides hitherto synthesized
by us all entail activation of a C-H bond of a substituent of
the 2,2′-bipyridine. Recently we have reported that reaction of
6-substituted-2,2′-bipyridines with electron-rich platinum inter-
mediates, typically (L)2Pt(R)2 (R) Me, Ph), allowed us to
achieve a different metalation, i.e., activation of a C-H bond
of a pyridine ring, with elimination of methane or benzene. This
interesting and still rare “rollover”17 intramolecular metalation
implies the formation of an N-C five-membered ring: the C-H
activation occurs on the substituted pyridine ring to yield
[Pt(N′,C(3))(R)(L)] complexes. Reaction of these methyl or
phenyl derivatives with HCl gives the corresponding chlorides,
e.g., [Pt(N′,C(3))(Cl)(DMSO)].
The N′,C(3) five-membered ring is rather robust, and a variety
of species, e.g., [Pt(N′,C(3))(Me)(CO)], 1b, [Pt(N′,C(3))(Me)(3,5-
Me2-py)], 1c, and [Pt(N′,C(3))(Me)(PPh3)], 2d, were obtained
by substitution of DMSO with neutral ligands: CO, b, 3,5-Me2-
pyridine, c, and PPh3, d. The easy displacement of DMSO is
promoted by the metal-carbon bond in trans position.
From compounds 1a and 2a the corresponding chlorides, 3a
and 4a, were obtained by reaction with HCl in acetone at room
temperature (reaction 2):
Here we report the synthesis and characterization of a series
ofterminalandbridgingplatinum(II)hydrides,[Pt(N′-C(3))(H)(L)]
and [Pt(N′-C(3))(µH)]n, all containing the N′-C(3) ring. The
nature of the neutral ligand L is crucial in driving the synthesis
toward a mononuclear terminal hydride or toward an oligomer
resulting from assembly of the highly unsaturated, 14-electron,
[Pt(N′-C(3))(H)] fragment.
Results and Discussion
As starting platinum(II) derivatives two cyclometalates,
[Pt(N′,C(3))(Me)(DMSO)], 1a and 2a, resulting from the ligands
1
One isomer is selectively formed: IR and H NMR spectro-
(9) (a) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions
of Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer
Academic: Dordrecht, 2000; p 560. (b) De Felice, V.; De Renzi, A.; Panunzi,
A.; Tesauro, D. J. Organomet. Chem. 1995, 488, C13. (c) Stahl, S. S.;
Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1995, 117, 9371. (d) Hill,
G. S.; Rendina, L. M.; Puddephatt, R. J. Organometallics 1995, 14, 4966.
(e) Labinger, J. E.; Heyduk, A. F.; Zhong, H. A. In ActiVation and
Functionalization of C-H Bonds; Goldberg, K. I., Goldman, A. S., Eds.;
ACS Symposium Series 885; American Chemical Society: Washington,
D.C., 2004; pp 250-263. (f) Johansson, L.; Tilset, M.; Labinger, J. A.;
Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 10846. (g) Lersch, M.; Tilset,
M. Chem. ReV. 2005, 105.
(10) (a) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.;
Fujii, H. Science 1998, 280, 560. (b) Young, K. J. H.; Meier, S. K.;
Gonzales, J.; Oxgaard, J.; Goddard, W. A., III; Periana, R. A. Organome-
tallics 2006, 25, 4734–4737, and references therein.
(11) Grove, D. M.; Van Koten, G.; Ubbels, H. J. C. J. Am. Chem. Soc.
1982, 104, 4285–86.
3
scopic data, in particular the large J(Pt-H) relevant to the
protons of DMSO (24.7 Hz (3a) and 24.2 Hz (4a)), support a
DMSO trans to the nitrogen atom. As in the case of 1a and 2a,
likewise in 3a and 4a the coordinated DMSO can be displaced
by CO, 3,5-Me2-pyridine, and PPh3, to give in good yields
3b-3d and 4b-4d, respectively.
1
Also in this series a comparative analysis of the H NMR
parameters, δ and J(Pt-H), accounts for the isomer formed,
which entails, as expected, a trans Cl-Pt-C arrangement. The
H(6′) resonances appear at very low field (δ ca. 9.4-9.8), as
usually observed when a chlorine is in close proximity,19 and
the relevant 3J(Pt-H) values fit in the order of trans influence
of the ligand trans to nitrogen, e.g., 3c 39.6 Hz, 3d 27.0 Hz.
Taking advantage of the series of chlorides available through
reaction 2, considering our interest in platinum(II) hydrides, we
(12) Bayon, J. C.; Kolowich, J. B.; Rasmussen, P. G. Polyhedron 1987,
6, 341–343.
(13) Minghetti, G.; Cinellu, M. A.; Stoccoro, S.; Chelucci, G.; Zucca,
A. Inorg. Chem. 1990, 5137–5138.
(14) Zucca, A.; Stoccoro, S.; Cinellu, M. A.; Petretto, G. L.; Minghetti,
G. Organometallics 2007, 26, 5621–5626, and references therein.
(15) Song, D.; Morris, R. H. Organometallics 2004, 23, 4405–4413.
(16) Stoccoro, S.; Zucca, A.; Petretto, G. L.; Cinellu, M. A.; Minghetti,
G.; Manassero, M. J. Organomet. Chem. 2006, 691, 4135–4146.
(17) (a) Zucca, A.; Doppiu, A.; Cinellu, M. A.; Minghetti, G.; Stoccoro,
S.; Manassero, M. Organometallics 2002, 21, 783–785. (b) Minghetti, G.;
Stoccoro, S.; Cinellu, M. A.; Soro, B.; Zucca, A. Organometallics 2003,
22, 4770–4777.
(18) (a) Botteghi, C.; Chelucci, G.; Chessa, G.; Delogu, G.; Gladiali,
S.; Soccolini, F. J. Organomet. Chem. 1986, 304, 217. (b) Azzena, U.;
Chelucci, G.; Delogu, G.; Gladiali, S.; Marchetti, M.; Soccolini, F.; Botteghi,
C. Gazz. Chim. Ital. 1986, 116, 307. (c) Bo¨nnemann, H.; Brijoux, W. Aspects
of Homogeneous Catalysis; Ugo, R., Ed.; Reidel: Dordrect, 1984; Vol. 5,
pp 75-196.
(19) (a) Byers, P. K.; Canty, A. J. Organometallics 1990, 9, 210. (b)
Sanna, G.; Minghetti, G.; Zucca, A.; Pilo, M. I.; Seeber, R.; Laschi, F.
Inorg. Chim. Acta 2000, 305/2, 189–205.

N,C “RolloVer” Cyclometalated Platinum(II) Hydrides
Organometallics, Vol. 27, No. 14, 2008 3417
deemed it worthy to attempt to synthesize new hydrido
derivatives with the N′-C(3) cyclometalated ligands. The
attempt, carried out by reaction of all eight chlorides, 3a-3d
and 4a-4d, with Na[BH4], was successful and allowed us to
achieve hydrido species. The outcome of the reaction however,
under the same conditions, is not the same in all cases, being
strongly dependent on the nature of the neutral co-ligands a-d.
Reaction of 3d and 4d with Na[BH4], in THF at room
temperature, provides, albeit in low yields (ca. 30%), mono-
nuclear terminal hydrides, 5d and 6d, respectively (reaction 3):
state and very soluble in THF as well as in CH2Cl2, where
however they are not indefinitely stable. Compounds 7 and 8
were easily identified as species with no neutral ligands by
1
microanalyses (C,H,N) and IR and NMR spectra. H NMR
spectra show a single ligand environment, consistent with
symmetric species; clear indication of the presence of a hydride
is provided by a resonance at high field in the hydride region,
δ ca. -14 (CD2Cl2). The pattern of the resonances is rather
complex, the main signal being flanked by symmetric sets of
satellites due to coupling with 195Pt nuclei. On the whole the
pattern is indicative of a bridging hydride: in agreement, in the
IR spectra no absorption is observed in the region of terminal
1
Pt-H bonds. At first sight the H NMR spectra (see Experi-
mental Section) allowed us to extract two 1J(Pt-H) values, e.g.,
compound 7, 1011 and 387 Hz, suggesting a hydride bridging
two 195Pt nuclei and being trans to a nitrogen and to a carbon
atom. A simple centrosymmetric dimer for instance could
account for the two coupling constants. The transoid disposition
of the cyclometalated N,C ligands should be attained in order
to prevent the hydride from being trans to two carbon atoms,
C-Pt-H-Pt-C. The highly unsaturated coordination of 7 and
8 however makes a dimer most unlikely: the EAN rule, on the
basis of a 16e closed shell, suggests each platinum connected
to two other platinum atoms and implies a closed oligomer.
Indeed a more careful analysis of the spectra allowed us also
Hydrides 5d and 6d can be isolated in the solid state, are
stable in air, and are very soluble in most organic solvents. They
were characterized by microanalyses and IR and NMR (1H and
31P) spectra. In the IR spectra a medium and sharp band around
2200 cm-1 is assignable to the stretch of the terminal Pt-H
bond. In the 1H NMR spectra the signal around δ -16 is
diagnostic of a hydride: the resonance, coupled to 31P, is
accompanied by satellites due to 195Pt (natural abundance ca.
33.8%, I ) 1/2) in the correct ratio for a mononuclear species,
3
to detect a long-range coupling constant, J(Pt-H), 39 Hz, 7,
confirming a nuclearity greater than 2.
To address the problem of nuclearity, 1H NMR and ESI-MS
spectra were obtained. The simulation of the spectrum of
compound 7 was carried out by means of the gNMR 5.0.6.0
program.20
1
1:4:1. The J(Pt-H) values, 1444 Hz, 5d, and 1424 Hz, 6d,
are large but comparable with those previously reported14 for
cyclometalated [Pt(N-N-C)(H)] derivatives, H trans to nitro-
gen. In particular they are similar to the value, 1460 Hz,
observed when N-N-CH is 6-phenyl-2,2′-bipyridine, which
gives a five-membered C-N ring as in 5d and 6d. The 31P{1H}
Both trimeric and tetrameric units were simulated. No
significant differences are observed in the spectra, and both fit
the experimental data. In detail we report here the simulation
for the tetramer species [Pt(N′,C(3))(H)]4; the spectrum of the
trimer is reported as Supporting Information.
1
spectra show a singlet coupled to 195Pt, with J(Pt-P) 2147
Taking account of the natural abundance of 195Pt, 33.8%, the
weight of the isotopomers A-E, expected for a symmetric
tetramer (Chart 1), is as follows: A, Pt4, 19.21%; B, 195PtPt3,
39.22%; C, 195Pt2Pt2, 30.04%; D, 195Pt3Pt, 10.22%; E, 195Pt4,
1.31%.
Hz, 5d, and 2174 Hz, 6d. Taken together the NMR spectra
unambiguously point, as expected, to the isomer having a trans
H-Pt-N disposition. Compounds 5d and 6d are new examples
of the capability of PPh3 to stabilize terminal hydrides with
nitrogen donors.4
The simulation was performed for each isotopomer sepa-
rately and, finally, for the entire system. The simulated spectra
of all the isotopomers are reported as Supporting Information:
the overall simulated and experimental spectra are shown in
Figure 1.
Isotopomer A (Pt4H4, 19.21%) is an A4 spin system with four
magnetically equivalent hydrides and gives an intense singlet
at δ -14.46. Species B, the major isotopomer (195PtPt3H4,
We can anticipate here (see later) that terminal hydrides can
also be obtained when CO is the co-ligand (5b and 6b), but not
directly through reaction 3. In the case of compounds with
DMSO and 3,5-Me2-py, 3a, 3c and 4a, 4c, under the same
experimental conditions, the reaction with Na[BH4] goes another
way (reaction 4):
With both the tert-butyl- and the phenyl-substituted cyclo-
metalates, 3a, 3c and 4a, 4c, the reaction occurs with extrusion
of the neutral ligand to give coordinatively unsaturated 14-
electron species, 7 and 8, respectively. The new materials are
obtained in good yields as red powders, stable in air in the solid
(20) Budzelaar, P. H. M. gNMR, version 5.0.6.0; NMR Simulation
Program; IvorySoft, 2006.

3418 Organometallics, Vol. 27, No. 14, 2008
Minghetti et al.
1
Figure 1. Experimental (a) and simulated (b) H NMR spectrum (hydride region, 300.0 MHz) of complex 7.
Chart 1
39.22%), is an AA′A′′A′′′X spin system. The spectrum consists
of four different sets of signals centered at δ -14.46: three
and C2 (195Pt2Pt2H4, 30.04% of the total) are both AA′A′′A′′′XX′
spin systems and were simulated separately. They are present
in a 2:1 relative ratio: C1, for symmetric reasons, has double
intensity with respect to C2. The simulated spectra consist of
several lines, corresponding to experimental signals, and indicate
that the major coupling constants (1J ) 1011 and 387 Hz) are
1
doublets (H trans to N, J(Pt-H) ) 1011 Hz; H trans to C,
1J(Pt-H) ) 387 Hz; H remote from 195Pt, 3J(Pt-H) ) 39 Hz)
and a singlet, H remote from 195Pt with no detectable Pt-H
coupling (likely H2 in species B, Chart 1). The isotopomers C1

N,C “RolloVer” Cyclometalated Platinum(II) Hydrides
Organometallics, Vol. 27, No. 14, 2008 3419
of opposite sign with respect to the minor one (3J ) -39 Hz).
1
2
Furthermore, a J(Pt-Pt) and a J(Pt-Pt) coupling constant
should be envisaged: the best fitting values appear to be greater
than ca. 3000 and ca. 200 Hz, respectively. These data are in
agreement with literature data for direct and long-range Pt-Pt
coupling constants.21 Isotopomer D (195Pt3PtH4, 10.22%,
AA′A′′A′′′XX′X′′ spin system) provides a minor contribution
to the overall spectrum, the signal being split into several lines.
Finally, isotopomer E (195Pt4H4, AA′A′′A′′′XX′X′′X′′′ spin
system), 1.31% of the total, gives a complex spectrum that,
however, can be ignored taking account of its small abundance.
Overall, the simulation indicates that, of the three 195Pt-H
coupling constants, the major ones (1011 and 387 Hz) are of
opposite sign with respect to the minor one (-39 Hz).
Furthermore, a 1J(Pt-Pt) > ca. 3000 Hz and a 2J(Pt-Pt) > ca.
200 Hz fit the experimental data.
Definitive evidence for the nuclearity of compounds 7 and 8
was achieved from mass spectrometric investigations. The
electrospray ionization mass (ESI) spectra of compounds 7 and
8, registered in CH2Cl2, show the molecular ion peaks at m/z
1629 and 1709, corresponding to the protonated tetrameric units
[7 + H]+ and [8 + H+], respectively: no significant peaks
consistent with a trimeric species are detected. Thus it seems
that, at least in vapor phase, 7 and 8 are present as tetramers.
Unfortunately, in spite of numerous attempts, up to now only
amorphous samples were obtained, unsuitable for single-crystal
or even powder X-ray structural analysis.
stretch and to the coordinated CO, e.g., compound 5b, 2179
and 2048 cm-1 (Nujol), respectively. Compared with the
corresponding chloride, 3b, νCO 2104 cm-1, the CO stretching
vibration is significantly displaced to lower wavenumbers in
1
the more electron-rich hydride. In the H NMR spectra the
hydride resonance at δ ca. -13 is coupled to 195Pt with 1J(Pt-H)
1459 Hz, a value consistent with a hydride residing opposite
the nitrogen atom. The trans H-Pt-N disposition around the
platinum atom was confirmed by a NOE difference spectrum
on complex 5b. In the spectrum, a contact between the hydride
and the double doublet at δ 8.08 due to the H4 proton shows
the hydride coordinated cis to the C(3) atom and, as a
consequence, trans to the nitrogen. Furthermore, the multiplicity
4
of the H4, (dd) is due to a small J(H-H) (1.3 Hz) coupling
with the hydride: a selective irradiation of the hydride signal
transformed the double doublet at δ 8.08 into a simple doublet.
To get insight into the relationship between the mono- and
the polynuclear hydrides, the stability of the oligomeric hydrides
7 and 8 toward neutral ligands was investigated by reaction with
DMSO, CO, 3,5-Me2-py, and PPh3.
With PPh3 both 7 and 8 react with opening of the oligomer
to give the mononuclear terminal hydrides 5d and 6d, respec-
tively (reaction 5) confirming the contribution given by the
Thus it seems that conversion of the polynuclear hydrides
into the mononuclear 16-electron species is easily performed,
under mild conditions, by π-acceptor ligands such as CO and
PPh3. It is not surprising therefore that most of the known
platinum(II) hydrides with nitrogen ligands, as noted in the
Introduction, are stabilized by phosphine ligands. At variance,
the ligands DMSO and 3,5-Me2-py do not react with the
polynuclear hydrides 7 and 8 under comparable conditions; for
example, 7 does not react in neat DMSO at room temperature
or with an equivalent amount of 3,5-Me2-py. Under more harsh
conditions (e.g., heating compound 7 in DMSO) and a large
excess of ligand, the equilibrium can be shifted toward the
1
phosphine ligands to the stability of the platinum(II) 16-electron
hydrides. The reaction occurs in very mild conditions, i.e., at
room temperature with a 1:1 molar ratio: compounds 5d and
6d are isolated in fairly good yields.
The analogous derivatives with CO in place of PPh3, not
obtained through reaction of the corresponding chlorides with
Na[BH4], were attained from 7 and 8 (reaction 6): The reaction
can be carried out by bubbling CO into a CH2Cl2 solution of
the hydrides at room temperature: in these conditions some
decomposition to metal occurs in the workup of the solution,
so that it is difficult to get pure samples. Remarkably, the
reaction occurs also in the solid state: analytically pure 5b and
6b can be isolated keeping the hydrides 7 and 8 under a CO
atmosphere at room temperature for several hours.
terminal hydrides as shown by the IR and H NMR spectra.
The NMR spectra, in particular, registered in d6-DMSO or in
d5-pyridine (deuterated 3,5-Me2-py not being available) give
clear evidence for terminal hydrides: a resonance around δ -14
is flanked by two satellites in 1:4:1 ratio. The large value of
J(Pt-H), ca. 1550 Hz, is in line with a terminal hydride trans
to a nitrogen donor. Attempts to isolate these new hydrides either
by concentration of their solution (DMSO or pyridine) or by
addition of hydrocarbons (n-pentane or n-hexane) failed: the
terminal hydrides have short lives in the absence of a large
excess of the neutral ligand, and the oligomeric species are re-
formed.
It is worth noting that the corresponding methyl complexes
[Pt(N′,C(3))(Me)(L)] (L ) a-d) are stable and can be isolated
in the solid state.
The new hydrides, 5b and 6b, were characterized as usual:
in the IR spectra two bands in the region 2200-2000 cm-1
(see Experimental Section) can be assigned to a terminal Pt-H
Experimental Section
The ligands were prepared as previously described.18 The
(21) Still, B. M.; Kumar, P. G. A.; Aldrich-Wright, J. R.; Price, W. S.
Chem. Soc. ReV. 2007, 36, 665–686.
compounds [Pt(bipyR-H)(Me)(DMSO)] and [Pt(bipyR-H)(Cl)(DM-

3420 Organometallics, Vol. 27, No. 14, 2008
Minghetti et al.
SO) (bipyR ) 6-tert-butyl-2,2′-bipyridine and 6-phenyl-2,2′-bipy-
ridine) were synthesized according to ref 17b.
H5′), 6.94 (d, 1H, JH-H ) 8.1 Hz, H5), 6.57 (d, 1H, JH-H ) 8.1 Hz,
JPt-H ) 45 Hz, H4), 2.36 (s, 6H, CH3 3,5-lutidine), 1.35 (s, 9H,
CH3 t-Bu).
All the solvents were purified and dried according to standard
procedures.22 Elemental analyses were performed with a Perkin-
Elmer 240B analyzer by Mr. Antonello Canu (Dipartimento di
Chimica, Universita` degli Studi di Sassari, Italy). Infrared spectra
R ) Ph, 4c: yield 94%, mp 235 °C (dec). Anal. Calcd for
C23H20ClN3Pt; C, 48.55; H, 3.54; N,7.39. Found: C, 47.97; H, 3.21;
N, 7.22. 1H NMR (CDCl3): 9.68 (dd, 1H, JH-H ) 5.7 Hz, JPt-H
)
1
were recorded with a FT-IR Jasco 480P. H and 31P{1H} NMR
39.3 Hz, H6′), 8.65 (s, 2H, JPt-H ) 44.4 Hz, Ho-lutidine), 8.27 (dd,
1H, JH-H ) 7.8 Hz, H3′), 8.05 (dd, 2H, JH-H ) 7.2 Hz, HoPh), 7.94
(td,1H,JH-H)7.8Hz,H4′),7.51-7.28(m,6H,H5+H5′+HmPh+HpPh+Hp-
lutidine), 6.75 (d, 1H, JH-H ) 8.1 Hz, JPt-H ) 40 Hz, H4), 2.40 (s,
6H, CH3 3,5-lutidine).
spectra were recorded with a Varian VXR 300 spectrometer
operating at 300.0 and 121.4 MHz, respectively. NOE difference
experiments were performed by means of standard pulse sequences.
Chemical shifts are given in ppm relative to internal TMS (1H)
and external 85% H3PO4 (31P). NMR simulations were performed
by means of the gNMR software.20
The ESI mass spectrometric measurements were performed on
a quadrupole time-of-flight nanoESI-Q-TOF instrument (Q-Tof
Ultima, Waters, Manchester, UK) equipped with a nanoelectrospray
ion source, calibrated in positive mode.
Synthesis of [Pt(bipyR-H)(Cl)(PPh3)], 3d and 4d. To a solution
of [Pt(bipyR-H)(Cl)DMSO] (0.140 mmol) in 15 mL of CH2Cl2 was
added 0.140 mmol of PPh3. The solution was stirred for 2 h,
concentrated to small volume under vacuum, and treated with
n-pentane. The precipitate formed was filtered, washed with
n-pentane, and vacuum-pumped to give the analytical sample as a
yellow solid.
R ) t-Bu, 3d: yield 88%, mp > 260 °C. Anal. Calcd for
C32H30ClN2PPt · H2O: C, 53.22; H, 4.47; N, 3.88. Found: C, 53.01;
1
H, 3.96; N, 3.97. H NMR (CDCl3): 9.81 (m, 1H, JPt-H ) 27.0
Hz, JP-H ) ca. 4.5 Hz, H6′); 8.23 (d, 1H, JH-H ) 7.5 Hz, H3′); 7.95
(t, 1H, JH-H ) 7.5 Hz, H4′); 7.79 (m, 6H, Ho PPh3), 7.41 (m, 10H,
Hp + Hm + H5′), 6.74 (dd, 1H, JPt-H ) 48.6 Hz, JP-H ) 2.7 Hz,
JH-H ) 8.1 Hz, H4), 6.46 (d, 1H, JH-H ) 8.1 Hz, H5), 1.28 (s, 1H,
CH3 t-Bu). 31P{1H} NMR (CDCl3): 23.65 (s, JPt-P ) 4308 Hz).
FT-IR (Nujol): 1098 cm-1 (P-C) s.
R ) Ph, 4d: yield 97%, mp >260 °C. Anal. Calcd for
C34H26ClN2PPt; C, 56.40; H, 3.62; N, 3.87. Found: C, 56.06; H,
3.88; N, 3.92. 1H NMR (CDCl3): 9.86 (m, 1H, JPt-H ) ca. 30 Hz,
H6′); 8.46 (d, 1H, JH-H ) 7.8 Hz, H3′); 8.0 (m, 3H, H4′); 7.85-7.78
(m, 6H, Ho PPh3), 7.48-7.26 (m, 13H), 6.90 (m, 2H, H4+H5
overlapping). 31P{1H} NMR (CDCl3): 23.63 (s, JPt-P ) 4286 Hz).
Synthesis of [Pt(bipyR-H)(H)(PPh3)], 5d and 6d. Method
A. To a solution of [Pt(bipyR-H)(Cl)(PPh3)] (0.10 mmol) in 30 mL
of THF was added a suspension of NaBH4 (0.20 mmol, 2-fold
excess) in the same solvent (10 mL). The mixture was stirred for
1 h, cooled, kept at 0 °C for 5 min, filtered over Celite, and
concentrated to dryness at room temperature. The residue was
recrystallized from acetone/n-pentane to give the analytical sample
as a dark yellow solid.
Synthesis of [Pt(bipyR-H)(Cl)CO], 3b and 4b. Into a solution
of [Pt(bipyR-H)(Cl)DMSO] (0.187 mmol) in CH2Cl2 (15 mL), CO
was bubbled at room temperature for 3 h, then the solution was
concentrated to small volume under vacuum and treated with
n-pentane. The precipitate formed was filtered, washed with
n-pentane, and vacuum-pumped to give the analytical sample as a
yellow solid.
R ) t-Bu, 3b: yield 77%, mp 117 °C. Anal. Calcd for
C15H15ClN2OPt: C, 38.35; H, 3.22; N, 5.96. Found: C, 38.38; H,
1
2.92; N, 5.78. H NMR (CDCl3): 9.44 (dd, 1H, JH-H ) 5.4 Hz,
JPt-H ) 34.5 Hz, H6′), 8.29 (dd, 1H, JH-H ) 7.2 Hz, H3′), 8.04 (td,
1H, JH-H ) 7.2 Hz, JH-H ) 1.5 Hz H4′), 7.66 (d, 1H, JH-H ) 8.1
Hz, JPt-H) 60.9 Hz H4), 7.47 (ddd, 1H, JH-H ) 7.2 Hz, JH-H
)
R ) t-Bu, 5d: yield 30%, mp 150 °C (dec). Anal. Calcd for
C32H31N2PPt · 0.5H2O: C, 56.63; H,4.75; N,4.13. Found: C, 56.70;
5.4 Hz, JH-H ) 1.5 Hz, H5′), 7.09 (d, 1H, JPt-H ) 10.2 Hz, JH-H
) 8.1 Hz, H5), 1.38 (9H, CH3). FT-IR (CH2Cl2): 2104 cm-1, s.
R ) Ph, 4b: yield 67%, mp 234 °C (dec). Anal. Calcd for
C17H11ClN2OPt: C, 41.69; H, 2.26; N, 5.72. Found: C, 39.82:
H, 1.43; N,5.36. 1H NMR (CDCl3): 9.49 (d, 1H, JH-H ) 5.2
Hz, JPt-H ) 34 Hz, H6′), 8.40 (d, 1H, JH-H ) 7.2 Hz, H3′),
8.08-8.06 (m, 3H, Ho(Ph)+H4′), 7.80 (d, 1H, JH-H ) 8.0 Hz,
JPt-H ) 61.1 Hz H4), 7.52-7.45 (m, 5H, Hm+p(Ph)+H5+H5′). FT-
IR (Nujol): 2112 cm-1, s.
1
H, 4.43; N, 4.13. H NMR ((CD3)2CO): 8.34 (d, 1H, JH-H ) 7.5
Hz, H3′), 8.18 (ddd, 1H, JH-H ) 7.8 Hz, JP-H ) 6.1 Hz, JH-H
)
1.2 Hz, JPt-H ) ca. 63 Hz, H4), 7.97 (td, 1H, JH-H ) 7.5 Hz, JH-H
) 1.5 Hz, H4′), 7.73 (m, 6H, Ho PPh3), 7.63 (d, partially overlapping,
1H, JH-H) 5.5 Hz, H6′), 7.50 (m, 9H, Hm+ Hp PPh3), 7.15 (dd,
1H, JH-H ) 7.8 Hz, JP-H ) 1 Hz, H5), 6.96 (ddd, 1H, JH-H ) 1.5
Hz, JH-H ) 7.5 Hz, JH-H ) 5.5 Hz, H5′), 1.39 (s, 9H, CH3 t-Bu),
-15.95 (d, 1H, JP-H ) 24 Hz, JPt-H ) 1444 Hz, H-). 31P{1H}
NMR CDCl3: 33.6 (s, JPt-P ) 2147 Hz). FT-IR (Nujol): 2204 cm-1
(Pt-H) s, 1097 cm-1 (P-C) s.
Synthesis of [Pt(bipyR-H)(Cl)(3,5-Me2Py)], 3c and 4c. To a
solution of [Pt(bipyR-H)(Cl)DMSO] (0.420 mmol) in 25 mL of
CHCl3 was added 4.2 mmol of 3,5-Me2Py (10-fold excess). The
solution was stirred and heated to reflux for 8 h, then it was
concentrated to small volume under vacuum and treated with
n-pentane. The precipitate formed was filtered, washed with
n-pentane, and vacuum-pumped to give the analytical sample as a
yellow solid.
R ) Ph, 6d: yield 30%, mp 173 °C (dec). Anal. Calcd for
C34H27N2PPt · 2H2O: C, 56.27; H, 4.31; N, 3.86. Found: C,
56.12; H, 3.72; N,3.71. 1H NMR (CD2Cl2), 8.46 (d, 1H, JH-H
)
7.5 Hz, H3′); 8.35 (ddd, 1H, JH-H ) 1 Hz, JH-H ) 7.8 Hz, JP-H
) 7.3 Hz H4); 8.14 (d, 2H, JH-H ) 7.2 Hz, HoPh); 7.86 (td, 1H,
JH-H ) 1.5 Hz, JH-H ) 7.5 Hz H4′), 7.68 (m, 6H, HoPPh3), 7.62
(d, 1H, JH-H ) 5.4 Hz, H6′), 7.56 (dd, 1H, JP-H ) 1 Hz, JH-H
) 7.8 Hz, H5); 7.43 (m, 12H, H(m+p)PPh3+H(m+p)Ph); 6.84 (ddd,
1H, JH-H ) 1.5 Hz, JH-H ) 5.4 Hz, JH-H ) 7.5 Hz, H5′); -15.91
(d, 1H, JPt-H ) 1424 Hz, JP-H ) 24 Hz, Pt-H). 31P{1H} NMR
(CD2Cl2): 32.8 (s, JPt-P ) 2174 Hz). FT-IR (Nujol): 2196 cm-1
(Pt-H) s, 1097 cm-1 (P-C) s.
R ) t-Bu, 3c: yield 95%, mp 210 °C (dec). Anal. Calcd for
C21H24ClN3Pt: C, 45.95; H, 4.41; N, 7.65. Found: C, 45.75; H, 3.92;
N, 7.65. 1H NMR (CDCl3): 9.62 (dd, 1H, JH-H ) 5.4 Hz, JPt-H
)
39.6 Hz, H6′), 8.62 (s, 2H, JPt-H ) 46.2 Hz, Ho-lutidine), 8.14 (dd,
1H, JH-H ) 7.8 Hz, H3′), 7.88 (td, 1H, JH-H ) 7.8 Hz, JH-H ) 1.5
Hz H4′ nv), 7.48 (s, 1H, Hp-lutidine), 7.23 (m, 1H,overlapping CDCl3
Method B. To a solution of [Pt(bipyR-H)(H)]4 (0.061 mmol) in
15 mL of CH2Cl2 was added 0.061 mmol of PPh3. The solution
(22) Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman
Scientific and Technical: Harlow, 1989.

N,C “RolloVer” Cyclometalated Platinum(II) Hydrides
Organometallics, Vol. 27, No. 14, 2008 3421
was stirred at room temperature for 10 min, concentrated to small
volume under vacuum, and treated with n-pentane. The precipitate
formed was filtered, washed with n-pentane, and vacuum-pumped
to give the analytical sample as a yellow solid. R ) t-Bu, 5d: yield
59%, R ) Ph, 6d: yield ) 75.1%.
Hz, JH-H ) 7.7 Hz, H4′), 7.36 (ddd, 1H, JH-H ) 1.4 Hz, JH-H
5.4 Hz, JH-H ) 7.7 Hz, H5′), 7.16 (d, 1H, JH-H ) 7.8 Hz, JPt-H
)
)
11.2 Hz, H5), 1.36 (s, 9H, CH3 t-Bu), -12.93 (s, 1H, JPt-H ) 1459
Hz, Pt-H). FT-IR (Nujol): 2179 cm-1 (Pt-H) s, 2048 cm-1 (CO)
s.
Synthesis of [Pt(bipyR-H)(H)]4, 7 and 8. Method A. To a
solution of [Pt(bipyR-H)(Cl)(3,5-Me2-py)] (0.53 mmol) in 120 mL
of THF was added 1.06 mmol of NaBH4 (2-fold excess). The
mixture was stirred for 1 h, cooled, kept at 0 °C for 5 min, filtered
over Celite, and evaporated to dryness at room temperature. The
residue was washed with cold acetone and filtered to give the
analytical sample as a red solid.
R ) Ph, 6b: mp 107 °C (dec). Anal. Calcd for C17H12N2OPt: C,
44.84; H, 2.66; N, 6.15. Found: C, 44.54; H, 2.81; N, 5.87. 1H
NMR (CD2Cl2): 8.68 (dd, 1H, JH-H ) 0.7 Hz, JH-H ) 5.4 Hz,
JPt-H ) ca. 18 Hz, H6′), 8.45 (dd, 1H, JH-H ) 0.7 Hz, JH-H ) 7.7
Hz, H3′), 8.24 (dd, 1H, JH-H ) 7.7 Hz, JPt-H ) 60.7 Hz, JH-H
)
1.2 Hz, H4), 8.12-8.04 (m, 3H), 7.58 (d, 1H, JH-H ) 7.7 Hz, JPt-H
) 11.5 Hz, H5), 7.51-7.37 (m, 4H), -12.92 (s, 1H, JPt-H ) 1458
Hz, Pt-H). FT-IR (CH2Cl2): 2196 cm-1, m, Pt-H; 2061 cm-1
(CO) s.
R ) t-Bu, 7: yield 70%, mp 210 °C (dec). Anal. Calcd for
(C14H16N2Pt)4: C, 41.28; H,3.96; N 6.88. Found: C, 41.36; H, 4.14;
1
N, 7.02. H NMR (CD2Cl2): 8.88 (d, 1H, JH-H ) 5.4 Hz, JPt-H
)
Synthesis of [Pt(bipyR-H)(D)]4, 9, and [Pt(bipyR-H)(D)(CO)],
10 (R ) Ph). To a solution of [Pt(bipyR-H)(Cl)(3,5-Me2-pyridine)]
(R ) Ph, 100 mg, 0.176 mmol) in 60 mL of THF was added 6.87
mg of NaBD4 (0.2 mmol 15% excess); the mixture was stirred for
2.5 h, then cooled at 0 °C, filtered over Celite, and evaporated to
dryness at room temperature. The residue was recrystallized from
acetone/n-pentane to give the analytical sample as a red solid.
ca.38 Hz, H6′), 7.97 (d, 1H, JH-H ) 7.8 Hz, JPt-H ) 49 Hz, H4),
7.74 (dd, 1H, JH-H ) 7.5 Hz, H3′), 7.56 (td, 1H, JH-H ) 7.5 Hz,
JH-H ) 1.5 Hz, H4′), 6.88 (d, 1H, JH-H ) 7.8 Hz, H5), 6.52 (td,
1H, JH-H ) 7.2 Hz, JH-H ) 5.6 Hz, JH-H ) 1.4 Hz, H5′), 1.32 (s,
9H, CH3 t-Bu), -14.46 (s, 1H, JPt-H ) 1011 Hz, JPt-H ) 387 Hz,
JPt-H ) 39 Hz, H-). ESI-MS (CH2Cl2): m/z 1629 [MH+].
R ) Ph, 8: yield ca. quantitative, mp >260 °C. Anal. Calcd for
(C16H12N2Pt)4: C, 44.97; H, 2.83; N, 6.56. Found: C, 44.75; H,
9: yield 62%, mp > 260 °C. Anal. Calcd for C16H12N2PtD: C,
44.86; H, 3.06: N, 6.54. Found: C, 44.55; H,3.14; N, 6.25. 1H NMR
(CDCl3): 9.01 (d, 1H, JH-H ) 5.7 Hz, JPt-H broad, H6′), 8.26 (d,
1H, JH-H ) 7.8 Hz, JPt-H broad, H4), 8.03 (dd, 2H, JH-H ) 6.9
Hz, Ho), 7.93 (d, 1H, JH-H ) 7.8 Hz, H3′), 7.58 (td, 1H, JH-H
7.8 Hz H4′), 7.51-7.40 (m, 3H, Hm+Hp), 7.21 (d, 1H, overlapping
CDCl3, H5), 6.55 (ddd, 1H, JH-H ) 5.7 Hz, JH-H ) 7.8 Hz, H5′).
1
3.02; N, 6.38. H NMR (CD2Cl2): 9.01 (dd, 1H, JH-H ) 5.6 Hz,
JPt-H ) ca. 36 Hz, H6′), 8.20 (d, 1H, JH-H ) 7.8 Hz, JPt-H broad,
H4), 8.03 (dd, 2H, JH-H ) 6.9 Hz, Ho), 7.92 (dd, 1H, JH-H ) 7.8
Hz, H3′), 7.62 (td, 1H, JH-H ) 7.8 Hz H4′), 7.50-7.38 (m, 3H,
)
Hm+Hp), 7.31 (d, 1H, JH-H ) 7.8 Hz H5), 6.59 (ddd, 1H, JH-H
7.5 Hz, JH-H ) 5.6 JH-H ) 1.5 Hz, H5′), -14.29 (s, 1H, JPt-H
)
)
1008.9 Hz, JPt-H ) 389.1 JPt-H ) 39 Hz, H-). ESI MS (CH2Cl2):
m/z 1709 [MH+], 1475 [M - bipy - H]+, 855 [M/2]+.
Method B. To a solution of [Pt(bipyR-H)(Cl)(DMSO)] (0.223
mmol) in 30 mL of THF was added 0.445 mmol of NaBH4 (2 -fold
excess). The mixture was stirred for 1 h, then cooled at 0 °C for 5
min, filtered over Celite, and evaporated to dryness at room
temperature. The residue was washed with cold acetone and filtered
to give the analytical sample as a red solid. R ) t-Bu, 7: yield
41%, R ) Ph, 8: yield ) 33%.
Complex 10 was obtained by bubbling CO at room temperature
into a CH2Cl2 solution of complex 9. 1H NMR (CD2Cl2): 8.68 (dd,
1H, JH-H ) 5.4 Hz, JPt-H ) ca.18 Hz, H6′), 8.48 (dd, 1H, JH-H
)
7.7 Hz, H3′), 8.24 (d, 1H, JH-H ) 7.7 Hz, JPt-H ) 60 Hz, H4),
8.12-8.02 (m, 3H), 7.58 (d, 1H, JH-H ) 7.7 Hz, JPt-H ) 11.5 Hz,
H5), 7.51-7.38 (m, 4H). FT-IR (CH2Cl2): 2061 cm-1 (CO) s.
Acknowledgment. Financial support from Universita` di
Sassari (FAR) and Ministero dell’Universita` e della Ricerca
Scientifica (MIUR, PRIN 2005) is gratefully acknowledged.
We thank Johnson Matthey for a generous loan of platinum
salts.
Synthesis of [Pt(bipyR-H)(H)CO], 5b and 6b. Solid [Pt(bipyR-
H)(H)]4 (0.014 mmol) was kept at room temperature under 1 atm
of CO for 72 h, to give the analytical sample in quantitative yield.
R ) t-Bu, 5b: mp 70 °C (dec). Anal. Calcd for C15H16N2OPt:
1
C, 41.38; H, 3.70, N, 6.43. Found: C, 41.75; H, 3.44; N, 6.52. H
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.
NMR (CD2Cl2): 8.61 (d, 1H, JH-H ) 5.4 Hz, JPt-H ) ca. 18 Hz,
H6′), 8.32 (dd, 1H, JH-H ) 7.7 Hz, H3′), 8.08 (dd, 1H, JH-H ) 7.8
Hz, JPt-H ) 60 Hz, JH-H ) 1.3 Hz, H4), 8.03 (td, 1H, JH-H ) 1.5
OM800027G